I have a thought... The three-prong "male" end of the fitting, from the handpiece. Might they have cut the back end off that -- the bit behind the collar, maybe intending to use the pronged base as a plant-on greeblie for something, and had that straight cylindrical remnant left over... and then sawed 1/16" slices off that (and flipped those) to glue onto the cheek? I don't know if they would have done just one on the initial sculpt, and then it was a cast detail in the ones we know, or glued individual slices on all six helmets after they were cast, taking some care to position them the same. Maybe there was a feature that was part of the initial sculpt that they ended up not liking and opted to cover it up with this infernally frustrating bit.
The thing I notice is that we have some nice close-up pictures o the inside of the right cheek of the Pre-Pro 2 specifically, from when it was being molded by/for Don Post. Yeah, there's tape covering that area, holding the visor in place, but there's no bump that would be the inside of the connector if it had any depth to it.
I need to check, but that three-prong cluster looks familiar, and I'm now going to be scouring the first couple movies looking for where that might have been used...
--Jonah