Found my AC " Bible"--THOSE AMAZING CAB FORWARDS by George Harlan.
No photos of back-up lights at all on the AC-4-5's. There is one photo of an AC-6 with a back-up light on its semi-Vanderbuilt tender, but it looks as if it was put on by SP and not at the Baldwin loco works, as it's the only photo in the book of a back-up light on an AC semi-Vanderbuilt tender.
Evidently the first back-up lights factory-supplied on the AC's were the built-in backup lights set in the rear of the big rectangular tenders of the AC-7 through 12. But the three earlier classes with the semi-Vandy tenders didn't have them unless at times SP applied them themselves.
So I'd say that our BLI's are modeled as delivered by Baldwin and not later modified (if at all) by SP.