quote from Dman
"The A10 was the only A series Datsun engine that did not have provision for PCV.

Have a look at the block under the exhaust manifold at the rear. If there is a metal tube sticking out to the side, or if there is a machined hole with some sort of plug in it, then it is NOT an A10.
If there is no tube & no hole in the block to allow one to be fitted, then it would indicate that it is an A10 engine.
There are other indicators but this would be the main one."

This is spot on as my early a12 has the rear breather tapped to make it look like the a10

Thanks 'D'. [refer to post #8]
There seemed to have been a lot of dickin' around when a quick look with a flashlight &/or a mirror would positively identify it in under 60 seconds.