Stainless steel sight glass adapter, complete with mountings. 
1/2" thread Mountings include a stainless steel washer, 1 x o-ring and a grooved lock nut.  
 
The adapter comes with a 1/2" bung so that a thermometer or thermowell can be added later if required.
 
Hole in top to mount sight glass is 3/8" (10mm) thread.