Is it an M54 female thread on the filter wheel, and M48 female on the OAG, or is the OAG M42?
This adapter might help you and would reduce the backspace consumption to 2mm. But the threads might need to be turned shorter if they foul on the pick up prism on one side and the filters on the other. There is an M54 to M42 version as well.
https://www.siderealtrading.com.au/p...-male-adapter/