Basic 1/2 inch BSP pressure relief valve.
Not calibrated in any way, just a simple (adjustable) spring loaded release pressure relief / safety valve with a lock ring to lock the adjuster in position.
Male 1/2 inch BSP thread.
BSP (British Standard Pipe Thread) sizes don’t match up with what you would measure using a ruler. Have a look at the BSPT link at the bottom of this page for more information about how plumbing threads are measured.