The unregistered mark Grimwade gives on page 274 as No 3871 is actually one of William Eley's four oval serrated bucklemaker marks of 1795 - 1796 given by him on page 405. Grimwade was not keen on bucklemakers, as was no-one else in 1990 and missed this .
However this mark does not appear to be one of these, but that an unknown maker. The Eley ones are very familiar to buckle collectors and are very regular, not rather amateurish like this . We have, I think, perhaps someone new in the firmament !