That’s a second generation 90 gearcasing. The very early ones (84-86) had a skeg which had a vertical trailing edge, not a raked one. The third generation casing from late 90s onwards has thicker walled housing which prevents to a certain extent the usual cracks these can develop.
They will all bolt up and work on any 90 (or 75/80/85 for that matter), The leg casting never changed on entire production run from 1977 75hp to 2017 90hp (the leg casting actually predates these engines as it was borrowed from the 55A)
You can also use a 75/85 gearbox complete, the only difference is that the internal gear change mechanism is a bit more basic than the 75C/80/90 box, but in practice you would never notice. The way to tell a 75/85 box from the 90 is that the splines on the gearshift shaft run are full length, the 90 box has 2/3 splines and the sealing Oring groove. That box has the preload pin which was deleted later on.
I wouldn’t be buying that locked up box though; They don’t lock up from lack of use, something has happened internally; It would cost the same again for a set of aftermarket gears, Dog clutch, forward gear taper bearing, drive shaft bearing, propshaft bearings, 6207 roller bearing, seals and new castellated locknut, plus reshimming. Propshaft May be bent too.