Consider asking over in /r/guns, some of those folks know a whole lot about that rifle.

There is even a subreddit for that:

/r/MosinNagant

In a word yes, it likely was issued, but unknown if it saw combat

According to data supplied from Karl-Heinz Wrobel's book on Mosinnagant.net (in German) Drei Linien Die Gewehre Mosin-Nagant (Three Line Rifle Mosin Nagant, see here for an explanation of the Line measurement) in 1941 the Tula arsenal made 982,211 M91/30's, 153,068 in 1942, and none in 1943 and 1944. Izhevsk made 891,180 in 1941, 2.873,697 in 1942, and 1,833,506 in 1943. For some reason many 43 Izhevsk rifles have survived to the point of being a cliche, and I personally have not seen as many 1942 Izhevsk rifles. I personally suspect by 1943, enough rifles had been made where many were able to be sent to troops away from the main fighting, but that's just speculation.

As for the history, books can and have been written. They were introduced in 1891 as a replacement for the Berdan II rifle, which had been originally been designed by Colt. The Mosin Nagant is really just a modernized Berdan II with somewhat modernized (by 1891 standards) bolt, and a five round single stack box magazine, and designed for a new smokeless cartridge, the 7.62x54r, which bears the distinction of being the longest issued cartridge in general use (The US Navy still issues a blank .45/70 for line throwing guns which may actually hold the record for longest serving cartridge).

The Mosin Nagant saw service in the Russian-Japanese War, WWI, WWII, Korea, Vietnam, numerous conflicts in the Balkans, and is still being used in Afghanistan. Essentially, any nation that received guns from Imperial or Soviet Russia has likely used the Mosin Nagant. The United States notably used the Mosin Nagant as a training and rear guard rifle in WWI, and also in the Allied Expeditionary Force in 1919 onward in support of the White Russians. These rifles were Remington and New England Westinghouse production purchased by the US Government when the Czarist government defaulted on the purchase in 1917.

In WWII, the Model 91/30, which was an updated version of the Model 91 Dragoon rifle (shorter than the standard model 91, with improved sights) was eventually replaced by the Model 1944 carbine, which featured a side folding bayonet and was considerably shorter than the 91/30 rifle. The last Mosin Nagant was made in Russia in 1948, when it was replaced by the SKS and AK 47 rifles. China, Poland, Hungary, and Romania all made Mosin Nagant rifles in the 1950's, using Russian provided training and equipment. Additionally, Czechoslovakia modified Mosins in the 1930's as carbines, and 1950's as sniper rifles.

Finland was another major end user of the Mosin Nagant rifle, and initially armed themselves with captured and purchased WWI surplus stock, which over the next twenty some odd years were rebuilt to various configurations, including new (and superior) barrels, sights, and stock configurations. Arguably, a Finn Mosin is the finest Mosin available.

Back to your Cabela's 91/30's, the ones I've seen lately have been coming out of Russia itself via the Molot arsenal, and are an interesting change of pace from the Ukranian refurbs that have been sold over the last 15 years or so on the US surplus market. These Molot guns have not gone through the same refurbishment process, and thus look more "original", including stocks that have not been varnished, more visible stock cartouches, unpolished bolt handles, and metal still retaining it's original wartime finish. Based on the condition of these, I believe that they were either rear line rifles, or simple cleaned up into better shape after the war and put away, without undergoing a full refurbishment like the Ukranian rifles did.

Russia gave away and sold many mosins, and it's not unlikely that the most used ones went first, while they retained the best for themselves. Based on the slightly higher prices, and more "original" quality, I believe we are starting to see the sale of some of these "better" mosins, with quite likely the worst of the lot being sold off first, although this is all speculation on my part.

So as much as I'd like to tell you that a given rifle saw combat, unless it has bona fide battle scars (I've seen them with shrapnel embedded in the stock, and bullet struck receivers for instance), paperwork proving it was taken from a combat zone, or similar provenance, all I can say is "It was almost assuredly issued, and there is a decent chance it was used in battle, but just to be safe, buy three or four like everyone else". Hope this helps, lemme know if you need more.

EDIT: I screwed up nested parentheses and such, but I'm far too tired to care. I have a Mosin to fend off grammar Nazis :p And remember, if you can only afford one rifle, they can both be a Mosin Nagant.

My roommate picked up one of those nagant specials at Cabelas -

It definitely could've been used in WW2, the damn thing was still "1940s" packed for storage- wrapped in waxpaper, packed in grease. His gunsmith said it looked as though it had never seen a cartridge- although 60 years packed in grease takes its own toll ( minor corrosion, a little wood softening in places)

it took almost 3 hours to clean the grease out before he took it out the first time.

its a nice rifle for $150 - plan on buying a cleaning Kit with it - be sure you are experienced enough to clean and prep it when you unpack it - or plan on paying your gunsmith to prep it.