The "best" serial number chronology is in the Second Edition of Walter's book, and it shows the Lewis Model beginning in production during 1901, at 55000, and going to 123677 in 1906. 91919 would still be 1903, but a Lewis Model.
Researcher is on spot here. #91919 is a Lewis model. The visual/mechanical differences: The Lewis model has a Top lever bolt as well as the under bolting. The number of screws/pins showing on the side of the frame varies. The Crass has five, the Lewis four.
Love those Ithacas,
John
