That's probably CG. Looks good enough that it could be real, but it's also very unusual as you've noticed.
Incidentally, clear soldermask seems to have been more common back in the day; Tektronix used this back in the 70s. Don't know if that's because it was a film process (with attendant poor registration tolerance -- not very suitable for SMT), or that that's how they preferred to do it. But at the same time, a lot of places went with bare copper (probably HASL board, followed by wave soldering?), which was probably a bit of a mess for various characteristics, but was probably cheap and fast...
I don't know offhand of anywhere that has clear soldermask.
Tim