A14 pistons are not only 3mm larger in diameter, but also have a larger wrist pin & have a bowl in the crown anyway.

All A10 pistons [for the Datsun 1000 model] are the same & yes, they are flat tops. The Coupe version of the A10 engine gains its higher compression from the different cylinder head that these engines have.

From what I can make of it the Coupe head is simply machined more on the head gasket face during manufacture as everything else about it is the same as the standard model head.