They didn't change much from when they were made by Borg Warner to brand new ones. I've stripped and built early ones (the one in my car was a prototype built for Holden) and brand new boxes still in the crate.
It's going to be hard to tell what sort of condition it's in unless you can drive it or strip it to see what it's like inside. even pulling the sump plug and looking for filings isn't going to tell you much because there are 2 big magnets in the bottom of the box that catch everything.
1st and 2nd synchros are triple cone design with carbon blocker rings so they work pretty well. The gears are all about an inch wide and very beefy. I've not seen any real dammage to them but the teeth on the synchro hubs get rounded and they can be hard to shift.
The weak link is the 3rd-4th synchros ans they are only single cone design. Corvette transaxle varieties got triple cone 3rd-4th but there's no easy way to swap them into the Holden boxes. The keys on the synhcro hubs shear off when shifting hard with big HP as they are only pressed steel. This causes the box to lock in gear. I replace them with billet steel keys I bring in from the US. I also ususally replace the aluminium 3-4 shift fork with the steel unit designed for the viper T-56 as it's a much stronger piece and wont flex under hard shifts.
Here's the keys out of Garretts box after I broke it at Shelbyfest last year
Cheers