Actually, there was an article a few years ago in handloader about this very topic. The author referenced 19th century sources of the term "Long Colt" including original cartridge boxes. As I recall, it was his conclusion that the Army was responsible for the terminology due to the confusion of .45 Colt vs. .45 Schofield in issue ammunition.
Maybe someone with the Rifle/Handloader CDs could do a search for the article.
