The same over 90 year-old fellow Camera Club member who gave me a NIKON FE and FM also gave me his two old Hasselblads that I had checked and cleaned for him ! A 1973 500 EL/M and a 1978 500C/M . The 80mm f2,8 Planar lens on 500C/M is in good condition and even came with a Hasselblad UV filter. One A12 back dated 1978 I managed to 'service' by taking off side panel and getting to lubricate all the winding gear and renewed a seal. The other worn A12 back 1981 had a screw that will not 'Budge' . The Mirror Bumper FOAM on BOTH is rotten and leaves 'Bits' on the mirror and at the moment I cannot see how to get under the metal flap in camera body that houses it. There was a better Focus Screen ion box included which I put into 500C/M . I have taken a few snaps to show you ---
I just had a look on google and there are several service manuals for your cameras available for free, just download, it may help getting under or removing the metal flap.