Pentax Spotmeter V screw part

I have A Pentax Spotmeter V which is working normally but has a part missing. It is the central threaded screw part that I believe anchors the rotating discs, keeping the ASA disc steady while the disc relevant to EV, aperture and f stop can be re-aligned for different exposures.
Does anyone know of a source for this part, preferably, but not exclusively, from the UK.
Pic attached and thanks