By "Fascist Manifesto" I assume you are referring to the "program of the Fasci di Combattimento" which was put together around June 1919 under the main initiative of syndicalist Alceste de Ambris and under the influence of other similar post-war programs, such as the one of R. Rigola for the CGdL.

First, it's worth noting that this specific program was, to quote De Felice "by all means, destined to remain a dead letter" - not only that, but, to give a significant example, when it was mistakenly published instead of the 1920 program during campaign for the administrative elections in Ferrara, there was such a violent reaction from the (largely agrarian) basis of the local fascio that those who had published it were forced to immediately apologize and clarify that "that one was the old program" - "the new one is a completely different thing".

So, in this sense, Fascism changed soon enough. Almost immediately actually - and the theme of the original purity, revolutionary and "anti-system", of Fascism, is in itself a myth internal to a proper fascist narrative (see for instance A. Pavolini's contributions to the idea of a "Fascist terror" and a vindication of fascism integralism during 1943-44).

A serious analysis of Fascism should not abstain from examining the character and nature of this change - but the assumption that Fascism was "very different in its original, true form" is neither a proper summary nor a conclusion warranted by such an examination. It is, more often than not, a deliberate or involuntary distortion introduced by looking at the Italian political system of 1914-22 under the quite nonsensical idealtype of "the left".

With that out of the way, the facts are:

1. The "program of 1919" wasn't a proper political program - certainly far, far, far from a "manifesto". It was a summary of general statements made on the press (most notably Mussolini's Popolo d'Italia) by certain adherents to the Fascio Milanese di Combattimento and by other influential figures (such as Alceste de Ambris) who were nonetheless not members of the aforementioned Fascio.

2. Those figures (again, De Ambris) had certainly given the program a strong "subversive" character which could not please the conservative groups, and especially not the agrarian forces. But, again, even at its inception, there were strong resistances from the other fasci to take this as a general program. In short, they believed it would have made their agreement with the nationalists and national forces impossible and therefore largely ignored it.

3. This fact was almost entirely immaterial for Mussolini himself, who had offered his patronage for the formation of the first Fascio on March 21^st and the constitution of a movement of the Fasci on MArch 23^rd 1919 - but then declined most responsiblities as far as their direction went (the Fasci had their own direction board, their own newspaper as well as their own funds).

4. Mussolini had no reason to commit to this special initiative, since it wasn't really that special. He had participated to various attempts to form collectives and concentrations of interventionist forces, on one hand opening to a "constitutional assembly of the interventionism", on the other attempting to maintain his ties with the groups of veterans and arditi who had come to identify with his newspaper during the war. For this reason he had to "ride two horses at once" - the democratic forces of the "left interventionism" and the nationalist ones of the "right interventionism" - the workers and the combatants - the people and the nation.

5. This was a difficult hand to play. Mussolini's progressive openings to the nationalist positions on the matter of Italian annexations after the war. His participation (with arditi and futurists) to the violent heckling of left-interventionist leader Leonida Bissolati on January 11^th 1919, his more or less indirect role in the sack and fire of the main socialist newspaper, the Avanti!, on April 15^th 1919, his constant (reciprocated) campaign against "official socialism", the "PUS" to use the favorite anti-socialist acronym, the "Italian Bolsheviks" - while popular with the national-interventionist field - made him almost a symbolic figure of the Milanese anti-socialist intransigent resistance. A figure, almost, of bourgeois resistance against the socialist threat (you can see for instance the reactions of contemporary observers from the conservative side to his position - for instance industrialist and miniser S. Crespi commented on April 17^th that "Mussolini was the leader of the anti-subversive reaction").

6. For these reasons, among others, Mussolini's attempts to promote his presence within a concentration of the democratic-interventionist forces failed - the USI (the interventionist Italian Socialist Union) rejected him obserrving how the inclusion of Mussolini's name in their lists was going to alienate all those of socialist inclination who were looking for an alternative to the Official Socialist Party.

7. Even then, Mussolini had no special reason to strengthen his ties to the Fasci di Combattimento as they were, by the end of 1919, marching steadily towards failure. Despite secretary general Pasella counting their numbers around 137 with 40,385 members (October 9^th 1919 - national congress) thanks to some very generous accounting, those members weren't very active ones (there is a rather funny exchange where the Milanese direction asks the fascio in Naples if they exist, and if they are doing anything), and at the end of 1919, after the impressive failure of the new movement at the National Elections, there were 31 fasci left in existence, with 870 members nationwide.

This was the character, political weight and impact of original Fascism - its trajectory - during its first nine months of existence, fascism was one of the many fringe movements without a definite collocation, without a precise program, without a cohesive direction (even with its small numbers, the Milanese direction went through various adjustments, in no little part because of the contrasts with the other fasci more open to collaboration with the nationalists), which fell in line with general themes of radicalism, social and political renovation and what we would call "anti-system" patterns. There is no surprise that it adopted certain republican, democratic or even syndicalist themes - and given Mussolini's attempts to build a large paltform for the upcoming elections, there is little surprise that he let things run his course.

When "original fascism" died, Mussolini remained, with the solid reputation he had built around himself, an influential political outled in his Popolo d'Italia and various ties to groups that were growing more active around the persistent political crisis centered around the occupation of Fiume - which offered him a good chance to further his role of "reference" for the national, anti-defeatist forces.

In the meantime, the local fasci, which had been abandoned in large part by the few members they had, were progressively filled back under the financial influence of the agrarian reaction during the 1920-21. At the end of 1920 the number of fasci had risen back to 88 with 20,615 members – to 317 with 80,476 members by March 21^st 1921 – to 417 with 98,399 members by the end of April – to 1,001 with 187,098 members by the end of May – and to 834 with 249,036 members by the end of 1921 (even more by May 1922, according to the Interior, 2,124 with 322,310 members).

I encourage you to examine these numbers, because they offer a good picture of the (actual - these are "true" numbers) size and political weight of Fascism when the movement gained political relevance.

Additionally, while I have written a lot more on Mussolini's personal trajectory during the war, I'll copy here a very brief summary of the political climate after the war - which may help explain better why Mussolini was not only positioned "to the right" already at the time of the formation of the first Fascio (and indeed certainly to the right of De Ambris) but why his attempts to appeal to "the left" failed spectacularly.