This is a stainless steel Thermowell used on some Raypak heaters. This includes the stainless steel well, a brass threaded piece and a brass ferrule.
The stainless steel tube is also available separately as part #007211F
Material: Stainless steel tube, brass fittings
Shape: Round cylinder open at one end, closed at the other brass threaded fitting is held on the tube at the open end using the brass ferrule
Stainless steel tube - length: 3-3/8"
Outside diameter: 1/2" (except for right at the open end) outside diameter: 9/16" (the tube is flared right at the open end so it's larger here)
Inside diameter: 15/32" brass fitting
Hex head on one end, threads on the other, round hollow center to match tube
Hex head is 7/8" from side to opposite side threads are 1/2" MPT (actual thread diameter for 1/2"MPT is about 3/4" length: 13/16"