Looking at the side cheek of the TrueTech filter wheel from the back
The female SC thread (48mm) on the side cheek facing the focuser had to be machined out to 50mm to fit the SX OAG unit. To do this the side cheek was accurately set up on a lathe using a 4 jaw chuck. To ensure the aperture was accurately centered, a mandrel was turned up to fit perfectly into the SC thread. The mandrel was then clamped in a drill chuck in the tail stock and used to align the cheek in the 4 jaw chuck by carefully adjusting each jaw to ensure the plate was perfectly centred. The aperture was then opened up (using fine cuts as the aluminium plate is quite thin) until the required diameter for the SX OAG flange was achieved.
Quite a bit of material needed to be machined out and the aperture ended up quite close to the edge of the side cheek. This can be seen in the photo. However, with still enough room to drill and tap the M2.5 hole near the top.