These Mk V cases have turned up before and all are of Robert Mitchell 1943 manufacture. Now, having looked at numerous 40 mm headstamps it would appear that the final acceptance pheon was not always added. Some Canadian manufactured cases such as 75 mm M18 and 76 mm M26 were sent south to the USA for filling. Both cases are fitted with American primers, manufactured I believe by The Lone Star Ordnance Plant (LS). The cases are the same year of manufacture and there's not a great difference in Lot numbers but have different primers.
"Notes on North American Equipment & Stores Section 9 Ammunition" (October 1944) states-
Primer, Percussion, 20 Grain, M23A2
This primer was the first push fit type to be fitted. --
Primer, Percussion, 55 Grain, M38A1
This is the standard primer in use by the Army for this ammunition. --
Unfortunately, no date is given for the changeover, but a table dated June 1944 shows M38A1 only being used for the 40mm, Shot, Fixed, A.P. M81.
My knowledge of 40mm L/60 is scant but as far as I can ascertain we didn't accept beyond Mk IV and a push fit primer was never approved. The cases could have been made for the US, but one would expect them to be stamped "M25" which was the U.S. designation. It is possible the Canadians were following the U.S. example of using push fit primers and trialling them. The next time I'm at the Archives, I'll have a look to see if there is any reference to the Mk. V case.
TimG.