I own one of these as well, and though Frank (yeah, I named my guitar) has been heavily modified over the 19 years I’ve owned it, it’s still my number one guitar, and always will be. It just has “the” tone.
List of mods:
Gibson 500-T humbucker (bridge)
New pickguard (all black instead of the stock “zebra”)
Bone nut
Pickups mounted directly to the body at a fixed height (added depth and resonation)
Had coil-taps, but went back to standard wiring
Pickups are wired “out of phase”, which gives a really interesting “open wah” tone when the selector is in the middle
Heavy-gauge strings
While I wouldn’t recommend these mods to anyone who is looking to maintain the “vintage” value of their guitar, as I’m sure I’ve killed any value mine may have had, they work for me, and really make the guitar a one-of-a-kind instrument. It does anything I want to do and more.

Sorry but this is probably a couple of years late

but, I bought one in Feb 1981 and I’ve not found another guitar that I’d want to replace it with.

The only information other than the original instruction leaflet that came with it that I can find is at http://www.vintageguitars.org.uk/sonex.php

Mine came with a hard flight case that even now maintains that distinct new aroma it had when I bought it. I’ve only ever seen three, two in separate music shops in Dundee, Scotland in 1981, one of which is now mine. and more recently a couple of years ago in Sound Control also in Dundee, they had one that they were using it in a sound booth to demonstrate amps, effects units etc. I’ll never sell mine its just irreplaceable

Dont know too much about those Gibsons, arent they the only model they made with a bolt on neck?
And doesnt the guy out of Razorlight play one?
Tell us a bit about the sound and feel of it, is it a maple neck?