The small arm that holds one end of the vertical encoder wheel in my Amiga tank mouse was missing causing the encoder wheel to move out of place. This small block is inserted into the side of the encoder housing and prevents the axle from lifting up. I didn't get a photo of the broken encoder, but the second photo shows the arm this replaces.

Please note there are several models of this mouse, so this may not fit yours.

The block should have a nice snug fit so it stays in place. One side has a small semi-circular cutout that cradles the axle. It's important this cutout is free of any blobs or stringing. This side should be inserted in first with the cutout down towards the axle, and seated fully.

You may need to scale the X and Y a percent. I printed this at 0.16mm layer height in PLA. As the part is very small, printing a few at once will allow more cooling time between layers and a more accurate part.