Dog
I must confess that i do not know just exactly what an A12a actually is, but if it is the B210 [120Y] version of the 1200 engine then it too has a 73mm bore as standard. The second oversize for these would then be 74mm & not 73mm + .040" even though there is not much in the diference.

With this bore size & a 70mm stroke, which is the standard 1200 dimention, the capacity is 1204.235296 cc [approx] Thats a long way short of 1270cc

To get 1270cc with the standard 70mm stroke requires a 76mm bore size [1270.208742 cc] which is where the stock A14 pistons come in. Boring the rods is no big deal, just a "few dollars more" Remember that phrase while making choices about your car & engine improvements. Believe me, they do add up.
The A14 piston is the right deck height & is a standard Datsun part, as well as being available from Repco etc.

Flat top pistons "can" raise compression from Half to a full point, depending on bore size, stroke & combustion chamber size, as well as the size of the original bowl, but be prepared to live with premium fuel at premium prices if you drive a lot.

Question.
Apart from custom forgings, where do you get flat top 76mm bore pistons? If you are using a 73mm bore [stock 1200 size] then use the Repco ones for the 1000 [A10] These are made on the shorter 1200 piston blank, but with flat tops. I used them in my wife's 1000 two door with A12 installed. Not a huge diference, but since the A10 & A12 pistons were the same price at the time, i figured the extra compression was for free.

Please tell me about the A12a engine as i am in the dark about this one.
Thanks........ Chris