a book (certainly pre 1800 which is the period I've been interested in) - many originals are now held by Manx Museum.
The marriage licences were on seperate bits of paper the couple presented to the vicar prior to the service - judging from comments the couple were issued with a copy of the entry in the register post marriage.
The filmed version is of a transcription (actually a very faithful copy) which is why all entries are in the same later hand - tho many might sign with a mark, they may well be able to read as writing with early pens was quite difficult
Baptims were it seemd entered some time post event - and usually after the church service if a private baptism was necessary