The 3/8" refers to the size of the threads for the boiler connection, and the 1/2" refers to the over-pressure outlet exhaust threads. In the F18 and Cafe Racer models, these are intended to be used in conjunction with a 1/2" Male x 14 mm straight barbed fitting and separate silicon hose. In the event of over pressurization, the exhausting steam would be directed through the 12 x 14 mm silicon hose, and then (eventually; it would go through a bunch of other stuff first) to the drain cup.
Sanremo part #: 10652094B.
Not adjustable
Designed to open with a calibration pressure of 2.2 bar
The steam discharge capacity is rated at 52.28 kg/hh
Includes an FKM (Viton) seal
Operating temperature -20/+200°C
CE/PED certified valve
Made of low lead content brass CW510L (OT57)