I've just checked the Second edition of "The Dancing Chain" by Frank Berto, and on page 109, Fig. 7.5 illustrates the 1931 French Cyclo Tank rear hub, which incorporates a freewheel body which can be separated from the freewheel pawls by hand. Iit may well be that mine is one of these, rather than the BSA pattern Cyclo Unit. Still doesn't explain the 40 holes, though.
Neil Foddering
Weymouth, England