These are my rough notes regarding the DEC BA23 chassis.
The BA23 houses the H9278A backplane.
On the back of the H9278A there is a BA23-A breakout board for RQDXn-series controllers.
I’ve not been able to locate any schematics for the BA23 enclosure.
The following manuals have useful information about the BA23 breakout board (and related components, such as the front panel):