Thanks for measuring the adapter for me James
. Measurement A includes the width of the accessory mounting surface, this needs to be removed I'm sure(without going to my observatory) before the adapter is bolted on. It describes this in the bellow link.
So (using the optical focal plane as the back focus of the camera) we have:
(37.338mm) - (6.985mm - accessory mounting plate) = 30.353mm + 13.8mm = 44.153mm that seams all right to me.
Josh