Could there have been an issue with fan vibration in these heads?
The Oxberry slide copy stand used a pin registered Nikon F2 and a Beseler Dicroic color head placed upside down on the bed.
The Dicroic head had the internal fan removed and a flexible plastic vent was attached to the head and routed to an external fan. The external fan
really reduced vibration.
I have one of these heads, standard as in not modified by Oxberry, and I have considered moving the fan externally.