Nice find! And for $20 you did well, although these bikes are generally worth "what a fool will give for it", as they say in our part of the world. As previous posters have already mentioned, bikes like these were produced from the 1930's onward, and are still being made. Excellent design for town use, and "vintage", you know. My gut says yours is from the seventies, or even eighties. Are those rims made from stainless steel? If you're looking for a production date, two things are worth looking into.
1. date codes on the parts. Hubs, cranks and brakes often have date codes stamped into them, which can often be deciphered with the help of the Internet.
2. the Atala logo on the chain case. I haven't seen that very often - if at all - and could point to a specific era.
Enjoy the bike!