Looks like what we (at BL back in the day) called a ‘Blanking Cap’ used when a header tank was added to a cooling system so the conventional radiator pressure cap was replaced with a blanking cap and the header tank had the pressure cap.
It was a job often done to Dolomites, Stags etc with overheating issues. Some models came off the production line with thus set up as OE.
Might be worth Googling Blanking Cap or ringing a company like Rimmer Brothers.
BR
Colin
