Unfortunately the STEP files for these items lack internal details that might show how the 500-series motor attaches to the back of the PG71 gearbox.
The spec sheets, dimensional drawings, and product pages (alliteration not intended) are also devoid of the kind of assembly information you're looking for with the following exceptions:
-- The RS-550 does indeed require two M3 mounting screws on a 25mm bolt circle. The RS-775 has M4 screws on a 29mm circle.
-- You can tell from
the picture that the spacer (am-2062) is made to fit between the main cylindrical gearbox housing and the plate that the motor mounts to. This would suggest that longer-than-normal M3 screws are needed to mount the 500-series motor.