In the thread some say the extension has been seen on NOR J50 cars as well. Could be inaccurate, or maybe a glitch and some were send to Norwood by mistake. Anybody's guess at this point.
When I had my 67 restored a few years ago, I asked American Auto Wire if the harness was still available, using the part number in the old thread I linked. They said it looked like their part number, but didn't see any designs. I think Factory Fit and AAW merged in the past, or Factory Fit is part of AAW? Fortunately my original was in very good shape, so I had it reinstalled with the new AAW harnesses.