I went and looked - the most "recent" patent date on mine is 1913 - and I know it was bought brand new in the 30's. So I guess they just kept using the older design. This is what throws many people off on dating machines, expecially Willcox and Gibbs because they always put the patent dates on the machine, and at the last, the patent was about 50 years old.
The patent dates on mine are
8/8/11
10/31/11
11/28/11
9/16/13
If your machine is missing the 9/16/13 date, then it's probably safe to assume it was made before that date - otherwise you'll have to go by something else to date it.