[SiriusHardware]

Thanks for the detail, you obviously know your 6502 very well. I'm hoping in the first instance that if there is a fault on one of the buffers it will make itself more obviously known and not be as subtle as you suggest.

If not, we'll try as you say, but as Colin already has a couple of replacements 'on shed' it may ultimately be more straightforward to replace them to rule them in or out - they are after all the same device, possibly the same batch, same age, performing a similar function to the address buffers which weren't working very well.