I think that the date letter is "b" for 1876 and that J.G is John Gilbert. His is definitely the mark on the knife blade with the "I" date mark for 1857. My only problem with the date mark on the forks is that I would have expected the punch to be an oval shape rather than the cut-corner rectangle. Hallmarks with duty marks from the letter cycle 1875 (a) to 1899 (z), in my experience, had oval punches with the cut-corner rectangles used for hallmarks without duty marks (starting from 1882 (h)). Here is an example of the "normal" 1876 mark:
and here are the 2 types of mark with the "h" of 1882:
It may be that your hallmark is an 1882 "h" mark (it is a little difficult to see exactly which letter it is), but with the wrong style of punch. 1882 seems to have been the first year when this dual system was in use so maybe somebody got confused and used the wrong one.
Phil