I bought this to connect my QHYCFW3 filterwheel to my Redcat51 telescope. The bolt-in spacers provided with the filterwheel were the width of the M54 adapters which meant I couldn't access the tilt plate screws on my Redcat51 when used. This spacer by ZWO is perfect, because the spacer is the width of an M48 spacer which means I can access the screws for the tilt plate on my Redcat51 without removing my camera & filterwheel after each adjustment. The back focus distance perfectly matches what I need so they got the length of this right. There is one caveat when used for this purpose.

The width of the M54 flange is about 10mm too short. My filterwheel, and likely the ZWO EFW 2"/36mm variant, has screw holes on the body of the filterwheel. The flange on the adapter is too short, so it doesn't cover these screw holes and if left alone, will let right through into your imaging train. This isn't a big deal, because I can cover the screw holes with black electrical tape but it would've been great if the flange was wide enough to cover them. ZWO make a shorter M54-M48 adapter which looks like it would cover the screw holes, but then I would need extra adapters to make up the back focus.