Peter, the sensor is 17.5mm back from the front making it the same as the newer SXV-H9C. The
handbook for the SXV-H9C shows a 25mm spacer is required to fit a pentax screw lens to the camera (page 6). However, the
HX916 manual shows a 27mm spacer to connect a pentax screw lens (page 5). I don't understand why they are different.
The newer camera has an M42/.75 thead on the front instead of an M42/1 thread as you have on that HX916.
Or,
these guys list a T-Thread(Female) to M42/1(male) adapter.