circuit 1 + 2 hybrid

One occasionally sees drop handle block instruments with a 'half height' back box. I've seen several photographs of such instruments in-situ on block shelves, but it was a while before I was able to get my hands on an example. Here it is:


The dial, clearly lettered GCR, is worth a closer look. The lower section of the dial is usually coloured green, but not in this case.

The instrument has a test label dated 15-8-1955. Removing the back box cover shows that the instrument includes features of both circuits 1 and 2:

Terminals A, B, Y and Z refer to the original commutator contacts and function as would be expected for circuit 1. Yet the commutator has been extended and additional contacts have been added, so as to replicate the Tyer rotary commutator of circuit 2. The photograph shows the commutator in the line clear position, so the LC contacts are closed, whilst LB and TOL are open. Terminals 4 to 9 refer to the added circuit 2.
The needle is wired into circuit 2.
This begs the question: why was this instrument so modified? At one level, the answer is: to use the BZR1 unit to provide block controls with a drop handle instrument (although one could also ask: why not just fit a Tyer commutator?). The instrument appears to have been used in this way, going by the wire count on terminals 4 to 9 (two wires on terminal 6 and one on each of the others).
Yet this does not explain why terminals A, B,Y and Z were retained. They do not appear to have been used in this case, as there are no remains of wires attached to them, yet they were obviously made available for use.
I wonder if this was an attempt to reduce the number of independent supplies required for the BZR1 unit, but that is speculation on my part. If anyone does know why, I would be interested to know!