This adapter is similar to the M54 to M48 step ring that comes with the ZWO 2" filter wheel, but it adds a thin lip to allow the ring to stop against the body of the filter wheel when used with accessories that are less than 54mm in diameter. The lip takes up 1mm of backfocus. There are two holes to allow easy removal with a tool since the lip is too thin to grab easily.
Male M54 threads
Female M48 threads