those who were appointed pre 1765 would appear in Libri Irrotulamentorum (books of oaths ) see www.manxnotebook.com/people/lbir_abv.htm
no James Grant there - thus if he was a collector of customs appointed by London then it would be post 1765 the 1st was Richard Betham who died in 1789 and whose memorial in Onchan churchyard states Collector in the Island 24 years