Cathy,
I finally reached my National Two Spool Machine. It's serial No. is H35106 and the slide plate does have date of August 8, 1911 and September 6, 1913 as the patent dates. It really doesn't matter to me when -- it's the machine that belonged to my husband's great grandmother, my son's great, great grandmother. i really, really wish these machines could talk. I'm sure they'd have almost unbelievable stories to tell!
Jeanette