This is a holder for two mosfets for the AM8 frame or similar alu extrusion frames. In my opinion if you want to upgrade your printer with mosfets it's cheap enough to do both even if it doesn't really matter.
The mosfets mounting holes have to be 42x52mm.
You will need 8x M3 screws to fix the mosfets to the mount and 2x M5 screws with T-Nuts to attach the mount to the frame.
For the AM8 you can place this mount behind the mainboard on the lower rail.