In trying to ascertain an answer to your question, I forgot that my GA645zi had a roll of Portra 220 in it.

Nuts.

No, there isn't a contact that you can just clean off. Yes, it's similar to others in that it has a motor advance, but under the covers they're all different in there. Yours might just need something internal cleaned off, but it has to be opened up by a technician. I know that there's somebody in the US who does some repairs to these, as I have seen his name mentioned on the forum. You'll have to do a bit of a search for it.

The mechanism appears to be strictly internal, with the internal computer keeping track of film position and the rest of it. It's not like these are a TLR, with a mechanical counter. Very likely there's some dust or atmospheric film on a sensor in there. Or corrosion between the last frame "beep" circuit and the counter. Just guessing, but it's all "not repairable this station."