Piston compatibility

I am shopping for pistons for a 1963 Norton Atlas 745 cc. Many suppliers of pistons list 1964 - 1969 as the compatiblity range. Anyone know why a '64 piston may not be compatible with a '63?

It might have something to do with over-boring. I seem to remember that the base flange used to crack off the Atlas barrels.

Wasn't there a change to the conrods somewhere early on there ?

Is this a UK piston supplier.
Almost all early Atlas's were exported, so there was no Atlas in the UK until about 1964...

Do you have the std bore dished top pistons ?

The original pistons were dished topped and 7.5 comp ratio. I replaced with 10:1 Comp ratio domed top. That was when fuel was 100 octane. Now I want to drop comp ratio back to 7.5 - 8.5. The 63 Atlas is an export (USA). Bore is +020. Most suppliers I have searched are in the UK.