I was able to obtain an AT8+ board in superb condition from eBay, serial number 8+03056 (so likely to be the 56th sold).
The breakout box and connector
When we first located the 78-way plug and socket the pair constituted the single most expensive components on the board. Before the production was outsourced to a factory in Scotland every connection here was soldered by hand and a test job was constructed to confirm that everything was correct.
PCB Front Side
The PCB is copyright 1989 but the build date is probably the 91/25, the 25th week (or day / batch) of 1991. The board is in lovely condition for being more than 30 years old! The firmware is V4.02, I’ll post a copy of it here in Intel Hex format soon.
PCB Rear Side
Note the initials of the hardware designer here, Frank Budd, employed as a contractor when new board designs were required.