Thanks for the suggestion.
Yes, 7/16″ does seem strange. So, it’s probably 3/16″.
However, trying to find a dome headed, nickel plated (or otherwise) 3/16″ 32 tpi screw appears to be virtually impossible to find (even on the internet).
Hence my request for help to locate a manufacturer/seller.
Mr Marshall is the man who usually knows these things ; I wonder if he might even have a couple in his box(es) of spares.