This plug board fit into the CPU 6510T and TRIPORT 6523A (or 6525A) sockets replacing their functions inside the 1551 drive.
It can be very useful if one of this very rare and expensive chip is not working.
(The other original chip can be sold to save a part of the cost..... )
The new CPU and VIAs are generic chips (not custom) and they are more common and more cheap than original,
so if you will have the necessity to replace them it will be simple and not expensive.
I have a question concerning the 6522... I see you have used the R6522, not the R65C22. Is that because the totem pole outputs on the CMOS version could cause problems?
If that is so then I assume one could use WDC's W65C22N which has that problem fixed. Right ?
)