I haven't seen enough pieces to be sure, but I attach a 1775 mark with no leopard's head. The Exeter Museum booklet, from which I think much of the Exeter section of Jackson's is taken, does say "After 1777-8 the leopard's head was no longer used."
This seems to go against the 1776 and 1775 marks featured here. The only way to clear things up is to find as many marks from this period as possible. This is a good opportunity to ask again for members to please post any 18th century Exeter marks they may have in the request in the Exeter Forum.
There was a Parliamentary report carried out in 1773 which investigated the assay offices of England. It may be that it was this which was responsible for the removal of the leopard's head from Exeter silver.
As for the 1781-3 letter I. In the Exeter Minute Book, at the meeting at the beginning of the 1782 assay year it is written: "Letter for the ensuing year continued."
I think this is most likely down to the partly interchangeable nature of I and J at the time, and not a lax attitude.