Thank you, much appreciated - a micrometer reading says 0.304 inches / 7.72 mm measures outside the thread barrel, overall lenght is 0.621 inches / 15.78 mm. the round, finger-tightening knurled top (black and metal) is 0.601 inches /15.28 mm in dia and 0.251 inches / 6.37 mm thick. Looks as if it could be the same screw as the swing back locking knob on the MPP Micro tech cameras - the design appears to be identical, black and metal knurled grip with a leather(plastic?) insert on the top flat. These are probably "hen's teeth" items, but if you can identify the thread at least I can pop a couple of non-original replacements in the tops as dirt excluders.
I live on the west coast of Norway, City of Bergen, beautiful, but totally devoid of all that's not metric. (Apart from pints in the 3 Irish pubs we have)
Thanks again, Mike E W