Thanks, that's what I thought.
Since posting the question I went and re-watched "Building my Dream Computer - Part 2" and realised that it says ~40K LOW RAM quite clearly on the board. And I was about to post that I had answered my own question when I went down a bit of a rabbit hole...
The serial number on the low RAM memory indicates that it's a 128K chip (which doesn't mean anything if those higher addresses aren't connected). Of course I needed to see if maybe they were connected and spent way to much time trying to find traces from the back of the board to the MEMORY DECODE and BANK SEL chips. I couldn't find them (but I had fun trying) and came back here.
