I think that you are probably right on about the “Roman Numerals” not being that, but rather a code. “VI” is not the only one. I have a picture of two 20 x 118mm cases in file, one headstamped “HA V 37 1939” and the other “HA V 36 1938.” It appears that “VI” was not the only designator.
Please note that both headstamps had other small markings, but not germane to this discussion and difficult ot type.
I also found a reply to a question made by the very knowledgeable German collector Manfred Beutter in ECRA Bulletin Number 493, item 493-8, page 4, that includes along with other text, a little table of headstamp use, but unfortunately with no pictures. The table reads:
HL and year (last two digits c.1894-1900
crown, complete year, and HL 1900-1911
CODE OF THE BRASS SUPPLIER, complete year, and HL 1912-1937
CODE OF THE BRASS SUPPLIER, complete year, and HA 1938-1942
It looks like our guess was correct.
Thanks for your insight on the supplier’s codes, not being Roman Numerals (well, they could still be Roman Numerals of course, as a code, but having nothing to do with the month of the year).