Wyatt_Earp74 wrote:
To the best of my recollection I’ve never asked iTunes to store purchases tracks as both AAC/M4As and mp3s. Does such an option exist?
Not to my knowledge (and tt2 will correct me if I'm wrong). The iTunes Store sells music as AAC files - that's it.
If anyone wants their music in a MP3 format then either they need to use iTunes to convert their iTunes Store purchases, in which case they would know that they had asked iTunes to do this for each file and on every occasion, or they buy them from a store that sells them in the format they prefer. Another (manual) alternative (in addition to tt2's suggestion) is the use of a third-party programme such as Audacity to create MP3 versions. Once again, the user would be aware of this - it's not an automatic or covert operation.
Why don't you simply delete the coverted folder? Better still, if you rename the folder with a leading X (for example) you will be able to recover it at any time in the future (whereas the recycle bin will automatically remove it after a set period, which may be before you're ready to delete it permanently). A leading X would cause iTunes to lose track of those files if they are actually listed in your library.
Once you delete or rename the coverted folder, you will be able to keep your eye on the folders to see if the coverted one is replaced by a new version. Of course, you could also check in the coverted folder to see if the creation date of these files offers any clue as to their origin.
As I type this, I've thought of something else: you can add one of the songs in the coverted folder to your iTunes Library, then use iTunes to check for details about that specific file that may provide further clues. Look at the following example song, that I use for experiments...
- If I recall correctly, this first version was ripped from my CD into iTunes, using iTunes (apparently back in 2010) and in AAC format:

- This second copy was also ripped from the same CD, but in wav format:

- Finally, this third copy was a conversion by iTunes, from the wav version, into MP3. iTunes did this only because I set it that task:

What do you find?