David might be a good person to ask.

I think that David may know or could probably look up exactly when the Burt Co. stopped producing in the 1970s TH&C chips. One source stated early 1970s, others mid-1970s, and one 1979. I think that mystery might soon be solved on its own as more Burt Co. order cards are posted on the ChipGuide.

I don't know if he would have special knowledge about when Paul-Son starting producing their H&C chips. I don't know who has access to their early order cards.