That's a Dura-Ace 8-speed Uniglide cassette.
If I'm not mistaken (again!) then you can also use HG sprockets on a UG-only body, all except the last (smallest) one that must be UG.
Hyperglide sprockets can be used if modified
by filing down the single wide spline. You're right about the smallest sprocket, but note that Dura-Ace used a smaller diameter thread than other Uniglide hubs (to allow an 11t sprocket to be used) making replacements very hard to find now.
The difference in thickness between 7 and 8 speed sprockets is minimal, you shold have no trouble at all mixing them up a bit.
For Uniglide there was no difference - only the spacers differed in thickness.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.