Look inside the enclosure where the MPU slides into.
On the back wall inside, there is the backplane board that the MPU plugs into when you slide it in.
Maybe take a photo of that circuit board inside, and we can tell you which version MPU replacement you need?
The two large, long white connector board headers are the most important ones to take note of and pretty determines what MPU is compatible with the backplane board.
Some versions use a long header on top, and a shorter header on the bottom?.... you'll see when you
look inside.
If they're different length headers (Long & short one), I would recommend what everyone calls a "504" MPU.
If the backplane headers are both long ones, then you would need an "1270 Enhanced" MPU.
I sort of outlined the MPU enclosure box with some yellow rectangles.
Also, I've attached a picture of a 504 MPU...look at the connectors on the bottom of the board.
Click on any photo to enlarge if needed...>>>