All airlines, including CO, used to allow smoking, both domestically and internationally. All seats had ashtrays in them - some may be just covered today. The domestic went away first, then later international. AF still may allow it - I am not sure. I am guessing it went away about 20 years ago.

The smoking sections were the last row of First Class and the back several rows of coach. The FA would just put a little sign speared to the seat back at the break indicating the start of the smoking section. Thus it could be adjusted as needed.

The problem was that if you were in a non-smoking row right next to a smoking row - the smoke still got you. Also going back to the bathroom - it would hit you. The smoking section was very smokey. International flights were the worst.