Scared Silly nailed it...
Nothing meaningful to add: Original T1s need an upgrade to the modern style piston as the old piston had sharp edges and there don't seem to be seats for that. The upgrade appears to already have been done, judging by the round rubber cap on the first stage.
That's basically the reg I dive. My second stages look like that but have the new housing that has a slightly larger bore, but that's really not important.
You got an amazing deal. As long as you use 30% nitrox or less (air in my case), this reg is hard to beat. This reg has more titanium in it than a T2X or T3, and has a steel piston instead of monel. I would not upgrade it with Monel parts to increase the nitrox rating. There are reasonably priced second hand T2X regs for that.