Hello Dave,
The reason for the board is a simple one, it was produced in the early 80's to replace the custom chip.
There was a fire in the factory that produced the original chips so the exspansion board was produced (very similar to the wallbox interface at the time) with a number of chips on it to do the same function.
My understanding is that it took almost a year before the factory started again with the original chips.
Regards
Alan
Alan Hood
ami-man
UK