Some of the A series Canon SLR's can also have a problem with a cover over the metering cell. Apparently it can go cloudy over time.
I don't know if this can be cleaned or if the cloudiness goes right through the material. I have my doubts that new replacements are
still available. Perhaps the cover could be removed during a CLA.

I found this out recently when my AE-1 was overexposing by 2 stops. I also checked my A-1 against a Canon EF, an F1-N and a T90 and it
was overexposing by 1 stop compared to the other three bodies. My A series bodies are both in exc condition and were last serviced
within the last 10 years. No mirror squeak. For the time being I'll just compensate via the film speed dial. I like using the A series bodies from
time to time as they are a lot lighter than my other FD's.