the 1 stands for book 1 - which includes all parishes - usually episcopal wills are arranged by parish (the order of which varies a little) but I've noted that in 19th century they can be more random.
There are quite a number of wills missing from Brian's index - generally in episcopal wills as he used the old filmed indices - these have now been updated for the wills up to 1800 but not past that point.
He also 'guessed' the film numbers based on the LDS titles - certainly the Manx museum films often break in mid year and thus I suspect Brian can be a film out - but the MM films may not reflect the current batch
In the next year the Museum intends to digitise all wills based on a new filming and many are currently in the process of conservation to enable this digitalisation - initially only for the new Fam History centre