Thanks for the input. I think I may have figured out my question after thinking through the progression of variations per HWS II a bit more:
-M1921 as originally specified: Tinned brass case, (3) holes in case side, inert tinned primer.
-As of 1942, inert primer eliminated (for ease of manufacture per HWS II), case still tinned (no mention of holes in case so I’ll assume they were eliminated as the tinned, primer-less case provided enough identifiers). Example of this shown above on left headstamped FA 42.
-An alternate was a tinned fired brass cased version with indented primer intact and (2) or (3) holes in the case. (I assume without the missing primer, the holes were felt needed here for identification).
-As of 1943, steel cases were used but were zinc plated to look like tin and had no primer. No case holes mentioned, so I again assume they were not felt needed, as the otherwise distinctive appearance was sufficient for ID.
-Also, as of 1944, un-tinned steel cases without primers were used and (2) holes in the case were again indicated. This would seem to imply the lack of a primer was not considered enough to distinguish it from other steel cased loads. An example of this is the item shown above on the right headstamped EC 4.
There were also copper plated steel cased and an un-tinned brass case version used. (These folks obviously had future collectors in mind with all these varieties!)
That’s a lot of assumptions on my part but as best as I can figure it and would be interested to hear of any examples that fall out of this format.