I have a Biltwel natural gas fired, gravity furnace, manufactured by Fraser & Johnston Co (785 Potrero Ave. San Francisco CA), serial number 43184, Size number 135. It's a 101250 BTU per hour model. I've been looking all over the internet, and the company was founded in 1928, and went out of business and/or was bought out in 1978.

It's still in running order, and has been retrofitted with a blower, so it's no longer entirely a gravity furnace... How old is this thing, really?

I suspect that it's showing it's age, in that it farts dust the first time it's turned on of a season! :p

Lol, actual age would be extremely hard to determine but an educated guess would be 35+ years old. My next concern with that furnace would be that it is distributing Carbon Monoxide to home if some fit it with a blower. I recommend replacing it with a new furnace or at the very least have someone with a Carbon Monoxide meter come in to determine if it is safe.

We have CO monitors in the house... They've never let out a peep outside of yearly testing.

I've watched the burner when the blower turns on, and the flames don't react at all. I suspect the exchanger is well built, and probably was modified to suit.