The work you need to consult is Lambert, B. Bibliotheca Hieronymiana manuscripta la tradition manuscrite des oeuvres de saint jerome, 4 vols. (1969-72). It discusses the complete manuscript traditions for Jerome's works.

I do not, unfortunately, have access at the moment. But someone else on here might, and can perhaps look up the answer for you?

If you have access to a university library, and can puzzle your way (or better) through French, ILL is always a good option.

These letters (technical term: sigla, singular: siglum) are a short hand used in the critical apparatus to refer to the different texts employed in compiling the edition, e.g. if most good manuscripts write Heliodorum in line 5 of the letter but the codex Turicensis Augiensis 41 (which has been assigned the letter Σ) deviates and reads Eliodorum then you find Heliodorum in the text and "5 eliodorum Σ") in the critical apparatus.
At the end of the day every editor has the freedom to decide which siglum to assign to each text but usually subsequent editors will try to keep things somewhat consistent by sticking to their predecessors' designations unless there is a good reason to deviate. Depending on the text in question different types of letters (upper case vs lower case, Greek vs Latin vs Hebrew, ...) may correspond to different types of source texts and letters can also refer to entire families of manuscripts or to reconstructed archetypes. Such things are usually discussed in the foreword to a critical edition.

The CSEL edition has the advantage of listing at the beginning of each letter the manuscripts used, so let's have a look at that: http://archive.org/stream/corpusscriptorum55stuoft#page/37/mode/1up

(maybe you notice that ς appears in the apparatus but is not included in the listing, under ς this editor collects readings from older editions).

s. is shorthand for saeculo/saeculi meaning "in the/of the century", so the editor used readings from one 6th century manuscript, one 8th century manuscript, two 9th century ones, ...

However, not all of these contain the full letter, notably Γ only contains a small part of it, from quidquid on page 37 line 18 to respicit on page 38 line 8).

So the first full manuscript of this letter used in the CSEL edition would be K, the 8th century Codex Spinaliensis 68.

I wish the praefatio would be a little more extensive (than the barely two pages it spans), e.g. the editor makes mention of the manuscripts that are of some worth (qui alicuius pretii habendi sunt), so it would be nice to hear which ones are in his opinion of no textual worth (usually because they are directly dervied from one of the good ones, just with more mistakes), and he doesn't even attempt to offer a stemma codicum (think of a family tree that outlines the relationship between the extant manuscripts).

(I guess it goes without saying but please keep in mind that older does not always mean better or closer to the original. Stuff like some 10th century monk happening across a - now lost to us - 6th century manuscript and making a faithful copy of it that is better than anything we have from the 8th or 9th century does happen.

That's why a good foreword - in which the editor outlines his thought process - is important and why I am a little frustrated with this CSEL edition.)