Just a note about the GMS:
Besides using a larger mirror to cover all lenses, the GMS does not have the foam pads holding the mirror on like the classic system, so they do not go out of adjustment with aging foam.
The foam pad are necessary on the older system because the mirror cage flexes when the mirror is brought down and the claw catches the corner of the mirror to hold it down. The glass mirror cannot flex like this without breaking so it need to be attached with a flexing pad. The GMS system does not need to flex.
My 503cx no longer focuses to infinity, which is a classic symptom of deteriorated pads.
My advice is to get a body with the GMS, it's one more thing that does not need to be service in an old camera body.
As I mentioned earlier, mirror is not larger on 501CM and 503CW. It just slides further than non-GMS bodies on a leaf spring behind the mirror.
Foam behind mirror can be tested easily by pushing mirror down with piece of plastic or something similar to see if there is any movement. If it springs back towards front of the frame, foam is working fine.
If it stays in the bottom of the frame, foam deteriorated completely.
Foam is certainly a weak point but it's very rare that camera needs service only for these foam pads behind mirror.
When this foam has deteriorated, it's the same situation with the foam that dampens the mirror and there is also the foam that sits in the groove where mirror frame mashes into focusing screen frame.
This other foam around focusing screen frame is the same on all 500 series mechanical bodies.
After enough time passes that this foam has deteriorated, body geometry is likely off in other places.
Body alignment specifications are plus/minus 0.03mm and mirror foam will drop it by 0.4-0.5mm. On average cameras coming in for service are in addition to this 0.3-0.4mm short. Few are closer and few are really bad.
If camera is aligned to factory specifications, it will focus accurately within few millimeters. It is very hard to adjust these without special tools.
Rear frame is stationary but if you look close to the middle of the back of the frame there is a dimple in the frame and mirror pivots on that small dimple.
Front side of the frame that is away from the crank (towards the lens) is sitting on L post on the right side that gets adjusted by very slight bending during service and keeps exact 45 degree angle.
Side closer to the mirror can also be adjusted by slightly bending the mirror catch. The purpose of the foam or spring is to return mirror to the same position after rewind.
All of this is different with motorized 500, 200 and 2000 series that are equipped with GMS system.