Very frequently, in a simple circuit which is what the metering systems of the 70s were, corrosion at contact points was due to the passage of electrons across dissimilar metals. Once, as a favour to a friend, I took apart a Kodak super 8 camera and found some internal corrosion, cleaned it off and restored it to her in a working condition. Probably what happened was the tech opened up the camera and did the same thing to determine the feasibility of repair. After all the repair company wouldn't want to quote on something that was going to be a monster to repair and lose money.
David.
(Nowadays it's a throw-away camera world)