The single-digit dates on U.S. cartridges, always representing either 1944 or 1955, were an economy move all the way around. It started with the removing of the “3” from 1943 bunters, causing the remaining “4” to be off-center. The same was true with the removal of the “4” from 1954 bunters, leaving an off-center “5”. New bunters for those years, 1944 and 1955, generally followed the “single digit” guideline, but the single numbers will be centered. I suppose this also represented a small savings in the cost of producing the bunters, but suspect it was mainly a case, again, of following the guidelines.
There were exceptions. There are a few in .45 M1911 caliber that are actually marked “44” but they are a little scarce. I have at least one, maybe a couple, from “44” and so-marked. I don’t recall if there are any two-digit “55” headstamps or not. I think I recall having a .30 Carbine bunter like that as well. This practice of single-digit dates was not limited to just the .45 M1911 cartridge.
I can only speak to the .45 and .30 carbine cartridges. I do not collect .30 M1906 or .50 BMG rounds, nor .38 Special.
Bunters were expensive to make at the time, and hundreds were needed for the quantity of ammunition being made in WW2. I do not know precisely why they continued the practice in 1955, as the Korean War was over. Likely, once again, just a matter of following existing guidelines and an economy measure. This single-digit dating did not exist before 1944 or after 1955.
It was not done, to my knowledge, on bunters from 1940. Occasionally you find a single-digit dated headstamp with a “shadow” second number from a bunter not totally ground out, and I have never seen anything other than a “3” “shadow marking.” If anyone has a headstamp with a visible, but ground down, digit other than a “3” from 1943, or possibly a “4” from 1954, please post a picture of it.
John Moss