Hi Martin,
Yes, the optical connection from the EFW to the camera is fine, I am using the M54M-M42F adapter for that.
The problem is the 4 empty 2.5 mm holes on the EFW 5x2, which allow light to come inside the EFW and cause exposure problems.
For large full-frame cameras like the ASI6200, there is no problem, because the camera body will cover those holes and the darkening rings can be used to seal the connection between the camera and EFW.
But for smaller cameras like the ASI533, the problem is the camera body is not large enough to cover those 2.5mm holes. Even if you use the darkening rings, those holes are not covered properly for smaller cameras. I am not sure if the camera body of the ASI1600MM is larger than the ASI533, so it may not have the same problem.
Gregg