Can't help with your maker - although I suspect it may be I.H upside-down.
The diamond mark was used for British registered designs - and belongs to the cycle in use from 1842 to 1867. The topmost "I" represents the class of material, beneath the letter "Z" is the year (1860), the letter "M" on the left is the month (June) the "6" is the day and the bottom "2" the parcel number.
The Public Records Office in London still has the books which contain the registrations.
I tend to concur with Neruda that the maker's mark is probably upside down. I have a copy of the Sheffield Assay Office Register 1773-1907 originally published in 1911 and subsequently reprinted in 1989. In it is a mark from October 18th 1866 which appears identical to the mark on your piece.
The makers name is John Harrison & Co (subsequently John Harrison & Co Limited.) They traded from Norfolk Works, 116 Scotland Street, Sheffield.