I think that, like most things, the answer is complex, and the answer changes with each manufacturer. The comments above are part of the answer, for decoration, to show that the letters represent words like U.S.C.Co. for “United States Cartridge Company”. There are also a lots of dots and dashes and stars and such that have no apparent use. Many probably do tell us something if we just understood what the code is.
Speer/CCI has used a large variety of dots and dashes in different positions to identify the CCI plant, or the contractor (like IMI) who supplied the cases. Most of these codes are known and have been documented in the IAA Journal. Recently Federal has used both “FC” and “.FC.” on headstamps. I seem to remember this was mentioned in the Journal but I do not recall the meaning.
Some of these variations may be just natural variation in headstamp design over time, but can help us date commercial cartridges that lack dates. Even letter styles can do this also.
A good example is RWS (Rheinisch-Westfalische Sprengstoff) in Germany during the pre-WWII years. They used a series of headstamps on 9mm Para cartridges including: “.RWS. 9mm”, “R.W.S. 9m/m”, “RWS. 9m/m”, “.RWS. .9m/m.”, “RWS 9m/m”, “RWS 1932”, “R.W.S. 1933”, “R.W.S. 1 . 39” and “RWS . 1 40”. During this same period Geco used even more variations on their headstamps including variations with “*” and “/” and “D”. Many of the Geco and RWS headstamps have similar formats which isn’t surprising since during most of the pre-WWII period, Geco produced the RWS pistol ammunition including 9mm Para.
As Olin moved 9x19mm production out of the Illinois plant to the Mississippi plant their caliber designation on commercial ammunition changed from “9mm” to “9MM”.
A close look at these headstamp variations raises all kind of questions. I have two apparently identical headstamps by IMI, but in one there is a small dash in the caliber marking and in the other there is a small diamond mark. Under a glass the difference is obvious but I’d have missed it had not a friend pointed it out to me.
Other “minor” differences also have meaning. I am told by a noted 7.9x57mm collector that one facility produced cases for a single caliber in two different facilities in their plant during the same time frame. One facility used letters with serifs and the other used letters without serifs!
It is easy to ascribe these type differences to chance or randomness in creating the headstamp bunters, but often there is an underlying reason.
British WWII 9mmP by ROF Hirwaun during WWII produced two basic headstamps on ball ammunition. The Mk 1 ball rounds were headstamped with the H^N code along with the date and the caliber. The Mk 2 ball rounds had the mark number added to the headstamp. In spite of only having one basic headstamp change during the war years, I have documented 13 distinctly different headstamps. Some have “Mk 2” or “MkII” or “Mk//” or significant variation in letter spacing or style like “9MM” or “9 MM” or “9M/M” or “9M.M.”. There are more variations if you count all the variations of spacing between the characters. I had written these off as random variations in bunter manufacture over time and a lack of quality control over bunter (or actually hob) manufacturer. I recently picked up a 48 round sealed box of Hirwaun rounds from 1944. I found eight distinctly different headstamp styles in the same box, and that is before I looked at different styles on the character 4 or minor differences in letter size, etc. If all these different headstamps were being produced at the same time it seems likely that there were quite a few case manufacturing lines operating at the same time and each had a distinctly different headstamp style. Since a single hob is used to produce a number of bunters (or so I have been told by guys in the business), this amount of variation is hard to explain by random variation or lack of quality control. I suspect somebody at Hirwaun once had a notebook which described these variations an identified them to particular production lines and or shifts or something similar. There is also variation in the ROF Blackpole production but not to the same extent as there is at Hirwaun, which seems to have produced a lot more 9mm than BE.
This is a tough hobby! The harder you look the more you find.