Abestos inside return vent?

Hey guys,

I started vacuuming the inside of my vents at my new house and I was just wondering if they used to put asbestos paper inside return vents. I know I have abestos paper on the outside of some of my vents (but not all), that I'm going to take some spray paint to.

The house was built in 1920 in southern Indiana. I've already sucked it all up in a vacuum, so I don't have photos.