welcome to the Forum....yes, you can pick up a MK II mag and reverse the button on the side to fit your older gun,, IF the button is reversed, for the early A54 frames and I'd guess yours is the later A100 frame,but never know over the years if anyone swapped out frames,,,,see that happen all too often,but I'd
bet your mag has the two slots ( both sides ) of the magazine chrome bottom, hold 9 shots , newer MK II holds 10 ..... you're good to go
Oh and if the bigger button is too wide for your frame , you can swap the button also, or trim the head of yours down in diameter........have seen some drag on the inside of the grip frame..........