That's a REALLY nice old saddle. I love it! The tooling is really cool. Unfortunately I have no idea what it is worth. I just admire old saddles, but I am not in the market of buying and selling them.
Before I signed in to see the photos, I was thinking, either it's a cheap knock-off or you measured the seat wrong. Because I have never heard of older saddles coming in 17" seat sizes. People and horses tended to be smaller back then (and I've been known to ride a 17" myself)!
So, I am thinking you probably measured the seat generously, because the fork angles forward, and you probably measured it up at the base of the horn to get a 17" measurement. Could that be a possibility?
I'm not even sure what the correct point to measure at is, but I would be pretty surprised if a 40's saddle measured a 17." But the way the fork tips forward, I could see you getting that measurement.
There's a lot of stupid out there!