UK motors all had a metal tag and no numbers stamped on the block, the stamping is an Australian thing. New motors bought here as spares had no stamped numbers on either. I bought two from BMC, years ago.
In NSW you used to go to the police with the block and a receipt for it, they would allocate a Police number and direct you where to get it stamped.
Number code in your case would be Q15****P, where 15 is year and **** is allocated number. P= da Fuzz.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.