This issue arose due to a concern raised by a Progress user, based on the following Knowledge Base article. And while looking into it, I discovered that we were not following the standard ATL protocol for the changing of the threading model.
Although I don't know the full history, it appears that the majority of the classes were changed in 2015 from single-threaded to multi-threaded model. However, this was done inside each class rather than in stdafx.h.
ATL allows for every class to be defined with a more generic model, called CComObjectThreadModel.
This definition is intended to follow a #define that is placed in stdafx.h so that you can change the entire threading model with one definition rather than having to change the definition within every class.