Originally Posted by
photogravity
They are MTB sized rims (is that 559mm?) and they are quite rusty.

I'll be on the lookout for some good deals on rims. I take it you're asking for a reason. You have something laying around?

No, just curious what size a tandem of this vintage would have if not 630 or 622. I would not have thought 559. 650B (584) was a popular size for custom ordered tandems in the 70s, but this bike is older and it's Italian.
If the rims are indeed 559, then you might consider the Compass 559x42 tire and nice polished, aluminum rims. There should be plenty of choices.