I have an alternative and very good use for your boss top and I am very pleased with it but I have found there is a difference between the Triple-M and post war Bluemel's boss. It is the aluminium ring (sometimes steel) for the one on the left below I was hoping for.
So, if anyone can help, I am still hoping to find one like this -
Very close to what I am looking for but your parts are Ashby rather than Bluemel's. It is confusing but laying an Ashby plate on top of the Bluemel's the problem becomes apparent.
... So not quite what I need but I am sure Simon would find a good use for them.
Thanks also to Terry, my Bluemel's boss is for a J2 .