Hmmm, I reckon it is just excessive crankcase pressure. Maybe the rings are not bedded in properly. The excess pressure has to get out somehow.

Our 1293A+ has an early Mini dipstick (coz I don't have the right one) just sitting in there, no rubber seal or anything, loose as- and doesn't leak a drop- even when wound out to 7000.
It only has the flywheel housing breather- no others, and it is just plumbed into the twin SUs 2 vac ports.

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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.