I thought that I'd add a follow-up to this old thread in case someone finds their way to it with a similar problem or question. I happened onto a Beseler plate #10-49524 that was the exact piece to solve this issue. I have 2 machines (an MXT and an XL) and quickly found another one on eBay so that now my 23CII and 45's can all interchange optics (I have different light sources on each).

This piece simply replaces the turret and allows the use of the standard 4" Beseler lens boards to be positioned correctly. Another thing that I discovered when mounting my 150mm was that the adapter on the turret (39mm to 50mm) was able to be used on the 39mm board, making its mounting easy and straightforward without buying a special board and/or pricey 50mm jam nut.