The threaded hole is slightly smaller than the standard modern 10mm so I suspect it is non-metric,(I haven't got the thread gauges out, it's a pain guessing on such a short thread.
Also, just for interest, here is an adaptor plate I made up to fit a Huret gear that normally fits on the r/h hanger to a Benelux dropout with a built in hanger like the one on the left. Works very well, Simplex to Campag fitting adaptor coming up next.
Richard
