Yes Dalmeny that's how it works, the 2nd 2 digits are the year. I said `66 in my previous post as that's when I did it.
Yes, yours has been stamped in `81.

When you bought a brand new block, short motor or long motor from BMC, it had no numbers stamped. You took your receipt for it to the local Police station and they allocated you a number.
They told you where you could get it stamped. You then had to return and let the cops inspect it was stamped correct.

I bought a new Morris 1100S short motor from BMC (Lancaster Motors) in 1968, it had no numbers either. I had to go through this Police number process, for this motor too.

I bought another Mini once with a 1275S motor in it, it had been Police stamped N851501P. So it worked for S motors as well.

[edited] for clarity.

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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.

There was an ebay ad a few months back with a bloke selling a "genuine" Cooper S ex-police car, he knew it was a Police car because it had a Police engine number.

Thanks, problem solved.

But, if that was a new block, was BMC replacing them with exact Cooper S blocks, as was use in the original fit? Considering the date (81) would there have been any cooper S blocks still around then?

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Dalmeny

Probably there still were some about, not necessarily in dealers (who were gone by then)- otherwise someone had decked a s/h block (stolen or not) and done a swifty with a receipt to get a Police number.

[edit] often when a block gets decked during rebuild (perhaps after several rebuilds) the whole number gets cleaned off.
Whether it's then legal or not to restamp the original number I don't know, but I suspect not.
(I make sure to leave the number pad alone when I deck blocks).

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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.

Thanks Dr,
I think the original question has been answered. So I'll stop asking questions that raise more questions.

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Dalmeny