They are actually more interchangeable than may realize. With a few little tricks.
I discovered the other night that not all "Mark I's" use the same magazine. Apparently at some point they switched the button from right to left. I've got one from 1960 with the button on the right, and one from 1974 with it on the left. If you look at the grip screws, and the bottom screw on the right grip is lower than on the left, you have an "A100" frame, and the magazine button should go on the left.
If the bottom screw on the right is higher than the one on the left, you have an "A54" frame and the botton should go on the right.
In theory, you can simply swap the button and use either magazine in either gun, but I have seen magazines that simply will not work in one gun, but will work fine in another.
The more I fool with these Standard Auto's the more they interest me, and they're inexpensive enough to not break the bank.
You can see the difference in the location of the grip screws in the pictures.