German WW2 recoilless cartridge?

Dear all, this is my first post so apologies if similar topics have been covered. I inherited an unusual brass cartridge from my great-grandfather, who fought in both the first and second world wars. At first I thought it was British, but the BK headstamp suggests it was German and manufactured by Hugo Schneider AG. The perforations suggest a recoilless shell, but I’ve been unable to identify it - can anyone help?

British artillery primer. Screws into the base of an artillery casing.

Ole

And it was made in 1949.

Never pick random letters and try to look them up in all sort of headstamp lists. This ends in 99% of the cases in miss-identifications.

Appears to be a No. 9 Primer Mk 3 used on 17-pr Gun, 3.7-in Mks 1-3 (reduced charge) 77-mm Gun (except APDS)
Manufactured by SHS - Samuel Heath and Sons Ltd. of Cobden Works, Leopold Street, Birmingham. (Brassfounders). Actually made in 1945, but filled in 1949.
BK is the “empty lot series”.
Filled by CY - ROF Chorley, Jan 1949.

TimG

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Very many thanks to everyone who’s taken the trouble to reply - I’ve learned more in the last few hours about this primer than in the previous 40 years combined. It’s much appreciated!