Hi Ken,
As you say, the smaller cameras have a 12.5mm flange focal distance, but the supplied m42 male to CSmount female adapter is ~7mm thick, which would make the rear element of the lens
at focus (I believe) about 12.5-7 = 5.5mm from the focal plane. Without modifying the ASI1600, by removing the front sensor cover inner aperture (black) plate and investigating or removing the entire housing front cover it isn't possible to get the rear element of the baby wide angle close enough (5.5mm) from the senor focal plane to achieve focus. If you chose to modify it you'd need a custom stepped adapter for the lens or custom front plate, but it could be done.
Roughly scaled image of the ASI1600 v 290 attached for reference.
Best
JA