Mike, Are you saying use both extensions?
Yes but, I'm just quoting what ZWO says gives you the 55mm needed. The camera is m42 thread and the flattener is m48. One of those spacer rings is a m42 to m48 stepdown adapter, the 16.5 I think. From looking at Matts pic I guess the 21 is an m42 extension so it has to go on the camera, then the adapter, then the flattener.
I had just finished installing the AT60FF flattener on my 60ED when I posted last night. But I used a different camera with a filter drawer so I used different parts to get the 55mm backfocus. It hasn't seen first light yet.