I'm no expert but it looks the same as my old 260z box only better, Supposedly fully rebuilt but looked like it was found at the bottom of the ocean. Dat2kman or PMC are experts on this.
To be certain you could swap the front mainshaft & countershaft bearings from the 260 gearset to the RB gearset, enlarge the shift rod holes and see if it slides together (the anti rattle shim will bind on the L series bell housing inner face, kinda looks like a power saw blade) or leave the shim off. If it does get the bearing holes enlarged and the inner face plunge cut to suit RB sizings. Stuart Wilkins motorsport has all the bearings, syncros, seals in stock.
I did an RB20det close ratio rebuild a not too long ago, it's so much better than the original boxes!