[Mondial]

Yes, that's pretty much it. Until about 1930-31, most AC powered radios used paper filter capacitors. Electrolytics were just being commercially developed, like the early Mershon caps. 

Any time you have a filter cap right after the rectifier of less than say 8 uF, I would recommend using a film replacement. Its especially important at this circuit location because the input cap sees the most ripple current. After the filter choke or field coil it not as critical because the ripple level is much lower.