Cubs ALL came with a 2-5/8" bore from the factory. M&W used to make a set of .090" oversize pistons for the Cub back in the '60's, but they were obsolete by the early '70's. Those were the biggest oversize I've ever seen made specifically for a Cub.
So I have no idea where those 3" pistons came from.
Any casting numbers on them anywhere?