This bike is beautiful. Nice job tracking it down. Now you need to tear it down and overhaul it and replace all the consumables.
The rear wheel is a helicomatic. You'll need to snag the right tool to pull off the cassette. Some people don't like the helicomatic system. It was ahead of its time and it had some advantages (easy as pie getting the cassettes on and off) and some problems. Here is some info on this
Maillard Helicomatic System from Yellow Jersey
The shimano 600/deore triple is beautiful. I really like the Huret derailleurs. I used a set when I did a cross country bike ride. The RD takes up a lot of chain. You will want to ease up a bit when shifting under a load with this. That's true for any RD but that's especially true for this one.
Oddly I picked up a 1983 Trek 620 yesterday; the bike looks like it came out of a time capsule.
