[Mr70]

They weren't only available OTC.
It started out as an RPO on 1970 vehicles sold brand new in the state of Calif.,parts of Nevada and a few other low lying areas in the southwest equipped with the GM regular production option <span style="font-weight: bold">NA-9</span>....aka EEC~(Evaporative Emission Control)..aka Charcoal Canister.

This was not part of the A.I.R. SMOG equipment as some say.