The ST850 is the replacement for the ST650. It uses two knurled screw-down caps, one for the valve sleeve and one for the insert. It does not use a clevis pin like the ST650 did.
The ST850 uses the same non-valve P17/B17 housings as the ST650. The valve sleeve goes into the housing first and is retained by the larger cap, and the insert is then slid in and retained by the smaller cap.
This transducer can also be used as a replacement for the older ST100/ST550.