Nice example. SM/C should be Singer Manufacturing Co. of Canada, the fuze maker, and 4AT the empty lot (see TimG's posts in
No 221 fuzes). Singer's plant at St-Jean (St. Johns), Quebec went over to ammunition work in WWII, including fuze inspection (
Community Stories: Keepsakes of Conflict).
CR/C on the time ring looks like Canadian Car Munitions Ltd (
British Fuzes and their makers & fillers monograms). CR/C is also tied to Cherrier, Quebec in
25 pdr propellant manufacturers, and Ingenium lists Canadian Car Munitions against the Cherrier plant (
Women manufacturing shells at the Cherrier plant). So filled at Cherrier, with 3/45 as the filling date, March 1945. Note that
Wikipedia lists Cherrier as a DIL shell-filling plant, so who ran it may be worth double-checking.
One caveat: the
WWII factories list gives "Sm/C" as Stormont Chemicals, a filling factory. Since the mark is on the body next to the Mark number, I'd still lean towards Singer as the maker.
I can't pin down the AB 2 / JY 2 marks on the rings. My guess is composition lot or ring inspection codes. Has anyone seen these on other Canadian 221s?