Hello,
Gentleman, let us take some time to consider some other options.
Joseph Mumford was quite possibly dead by the time this bottle cap was assayed. The forum has some detail of his likely demise in 1724 or 1727, this is to be seen at:
http://www.925-1000.com/forum/viewtopic ... 74&t=25750" onclick="window.open(this.href);return false;
A close inspection of the "IM" reveals the possibility of further detail above the letters, and perhaps also the exsistance of a pellet between the letters.
Bearing in mind this possible extra detail, take a look at the mark of James Manners, noted by Arthur Grimwade and recorded as No. 1515. It was registered in 1734.
Bassett