I’ve wondered the same thing in regards to the shoulder position. I can only assume that since they’re part of the same series that they’re used in the same gun(s). From the photos I’ve seen of the A2’s, they appear (from looking at a picture…) to be dimensionally identical to the A3’s. I’ve never seen a plain M18 though.
On another note, according to MIL-C-20367G, the M21A1 is simply the designation for the case; not the entire cartridge. There’s also a M21A1B1 out there somewhere. Just for reference, the drawing number for the M21A1 cartridge case is 7553197. The “M18 Dummy or M18A3 Drill cartridges” are specifically called for use in M10 links (M24A1/M3-Type guns) when testing the links for free-hinging or testing the various linking machines per MIL-L-11536D and MIL-B-21341. As of yet, I haven’t found a reference specifically calling for the use of M18A2 dummies.