The LDS Arch and Epis indexes which Brian used are not later LDS transcriptions but films of the mostly hand-written original indexes held in the MM. They were filmed at the same time as the wills themselves, and are listed by year and by parish, so take a long time to find particular names, hence Brian's amalgamated lists. There were 19 years omitted from the Epis (filmed) indexes 1679-1697, and he used the filmed wills to compile his own index to add to the MM ones. Apart from these 19 years he went by what was written in the MM indexes which accompanied the wills.
It was an ongoing project, and he hoped that people transcribing wills would alert him of any indexing errors.

Sue