It would be great if ZWO could provide a 16.5 mm length T2 (M42x0.75) spacer, with male threads on one side and female on the other.
Then, for most reducer-correctors, which have been designed for 55mm back-focus, using a ZWO ASI 183C or ASI 1600, etc,, that have the 6.5mm setback. one could get this distance exactly.
Currently I am using an 11.55 mm spacer (that came with my Celestron OAG), a 5mm spacer, the filter wheel (20 mm) and a 12 mm T-thread spacer to get:
11.55 + 5.0 + 20.0+ 12.0 + 6.5 (sensor set back in camera) = 55.05 mm
The 183 and 1600 come with a 16.5 mm spacer, but it has 48 mm threads on one side and T2 on the other. So it doesn't work with the vast majority of people who have T2 threads throughout.
It is the 6.5 mm sensor setback that causes the issue. There are not many spacers with 0.5mm increments. (Though Baader does have a 7.5mm spacer, which would then require a 4mm and a 5mm spacer to get to 16.5mm, as a 9mm spacer does not exist either).
My two ZWO cameras* also came with a "zero-length" M48-to-T2 thread adapter. that is, it does not add to the length of the 16.5mm spacer.
So the supplied 16.mm spacer now works perfectly.
*I have 4 ZWO camera so far. I may buy two more.
Thank you ZWO!