That's easy to verify -
@oniongal did you miss any photos? I.e. do you remember making photos that ended up as blank spaces on the roll, or did you get to make ca. 20 photos on a 36 exp. roll?
Personally I don't think it's that; my guess is that the mirror is stuck. The sticky shutter issue
@loccdor mentions is also possible, but it's usually evidenced by capping on some frames. It's pretty easy to detect most of the time; if we could have a look at the frames that did end up being exposed OK, it might be able to tell whether there's any shutter capping going on.
If it's indeed an intermittently stuck mirror, I'd start by inspecting around its edges for any signs of degraded foam or other sticky junk that might hold it down. The mirror requires very little force to lift, and consequently only minor contamination can hold it down.
As to servicing; I think it's safe to assume nobody has bothered to service this camera in the past 25 years, even if they would have tried to locate the nonexistent entity that would be in a position to do so.