the light fixture is what would date this most accurately. Singer didn't offer the one like that for very long. They seemed to like upgrading the light more often than anything else. I'm in the office so can't dig through my manuals to see what date range this one was offered.

I would agree. I have my Grandmother's 66-6 from 1928 and it has the same light and same cabinet.

I am not an expert on sewing machines like many of you here are, but isn't this the light and light switch on the back of the machine?