You're in the UK, right? Im in the Netherlands so we're only 1 hour apart timewise (in your advantage as well)

Ive started a layout based on these replacements, but its going to be pretty big with al these ics and a lot of jumpers. See if I can upload a picture of the piezo tonight

I've checked and it is an LFSR. Interestingly, because you can't get tap 11 on the 4006 (only 4 stage blocks, and a couple of single stages) it uses taps 17 and 18, giving a degenerate cycle of 253,921 bits instead of the full 262,143 you'd get from a "perfect" 18-bit LFSR. Close enough, without needing another chip. Roland used 2 x 4006 in the TR909, I notice, for a much longer register and less noticeable repeat.

Now, how fast is that NAND clock running?