Isaac Haines was a person, a Lancaster Pennsylvania gunsmith, who worked from before the Revolutionary War through perhaps 1800 or so (don't have the book in front of me). It is a shame that the kit suppliers do not give customers even a glimpse into the history of original guns. There are several fine examples of Isaac Haines rifles known and 3 of them, I think, are in Shumway's Rifles of Colonial America, volume 1, toward the back of the book.
The Isaac Haines kits being offered are simply Golden Age (1780-1800) generic Lancaster styled stocks with common Lancaster styled furniture, and generally, a Siler or Golden Age lock from Chambers and associated parts. Sometimes the kits will have a blueprint that shows Isaac Haines' style carving, etc. He was a fine carver and his work leaned toward sophisicated more than folksy.
Since Isaac Haines worked for several decades while styles were changing, then variations in parts are normal. But the generic kits which have buttplates perhaps 1 and 3/4" wide with some dish, and barrels 1" or narrower at the breech, should, stylistically, have long barrels, because the rifles correspond to the 1790's or so. Shorter barrels than 42" were rare in the 1780-1800 time period.
Investing in books illustrating original arms, and visiting museums that have originals, are very worthwhile investments for the rifle builder. It is a very big mistake to use catalogues as sources of information about originals. Many of them mis-label and mislead so often it seems they must be doing it intentionally. I recently saw a Bucks County styled rifle advertised at one of the big catalogue companies' websites where they sell custom guns, labeled as a "York County" rifle.
Long story short- what is sold as an Isaac Haines rifle just means 1790's Lancaster, more or less.