There's a vintage machine sub-forum that you might check out - lots of very knowledgeable people there and someone might be able to guide you to a manual and help with figuring out its age.
I love old machines - sounds like you found a good solid workhorse! Does it have a metal body? Just think how much you'd have to pay to get similar quality new, now. If you could even find it.
Whenever I go to thrift stores, the first thing I do is check for old machines and accessories. I'm always hoping to find a treasure!
