Obviously the machine could not have been made before the newest date on that list. Sometime you can do some research and find one that has a later list of patents (like the next patent being 1906 or something) you can then use that to narrow down the date range of your machine.
For example For the early Howe machines there is a specific list of patents that were stamped onthe plates. so, when you find a machine that has only the first 4 patent dates, then you know the machine was made between that date and the next patent date. (because otherwise it would have included that next date...)
makes sense?
P.S. you might want to do some research on that treadle base design. I thought that the sheet metal formed legs were begun in the 30's-40's, but I could very easily be mistaken. It may not be the original.