One trick pony - but it does the trick well
My scope's flattener has m48 (48 x 0.75 mm) threads, and requires 55 mm backfocus. I have a Sony a6100. This adapter connects the two with perfect backfocus in one step, which is great. Less great is the fact that you have no flexibility to put anything else in the imaging train, like a filter. I'm alright because my flattener holds a filter anyway.
If your equipment ends in m42 threads and requires 55 mm backfocus, and you want to use this adapter and an m42-m48 adapter, you need to make sure the smaller adapter adds zero length. Such an adapter might be difficult to remove.
Backfocus is not an issue if your camera is going to be at prime focus. Typically you are going to need a bunch of spacers to get that working.
Reviewed by: Ronald Stewart from Vancouver on 5/9/2021