The previous thread on the .236 USN reminds me of a recent find in this caliber. It looks like a normal ball loading with a tinned copper jacketed 112 gr. bullet but it bears a ringed oval brass primer stamped with a “O” in the center.
WRACO Volume II p. 191 says: “Smokeless powder ledger indicates ‘Change lenght of primer cup 6 m/m and all similar. Cup (6 m/m) is now tinned & has no ring’ 9/22/98. (Author’s note: The mention of a ring in the first segment is a circle marking on the cup which was used to denote experimental. This does not refer to a ring crimp.)”.
An empty primed case using this primer and with a deep cannelure on the neck is also mentioned in “History of Modern U.S. Military Small Arms Ammunition, Volume I” p. 47.