1975 So Cal "installed Smog Thing"

I have been transcribing my dads note books from the maintenance of his 67 Dart convertible.
I'm up to 1975 (100 lines in an excel spreadsheet) and he has an entry "Installed Smog Thing"
I have a very vague memory of some device that California required for a time till it was found to be illegal to require or something like that. It was added to cars at the time but don't have more than a fleeting memory of it.

Anyone have any knowledge of what the "Thing" was?

I don't think it would be a stock Mopar thing, nor was it referring to a CAP part.