I have recently purchased a a65 in a a50 frame would be a factory bike?
Depends on the year, PRE 66 bikes would have had A50 numbers. It's the letter that follows that number B,C, D which tells us a Little more about the original designation.
If it's 66-onwards then an A50 frame would have had an A50 motor in it from new.
Apart from the capacity there's not much between the 650 and the 500 as most parts are common between the two.