Hi Greg, dug out a couple examples....
The tape is 2" wide black gaffers tape (not duct tape) and it appears to be torn from a roll and originally covered the lamp socket hole. I checked about 4 examples that still had the tape and it varied in length from about 2 1/4" long to 3 1/4" long. It appears that the taillight housings were shipped with the tape covering the hole, and when the car was built, they peeled the tape off and stuck it to the housing. I have about 5 or 6 used housings with the tape still stuck on them. I took a pic of the original housing off my 10-30-69 CJ car, The white stuff on the bucket is not paint, it's the zinc finish oxidizing. I have some nicer examples, but that housing is from a Metuchen car built 9 days after yours.
The second pic is of an NOS taillight housing with the tape still covering the hole. I got about 6 of these in the early 1980's in a lot of parts I bought from a guy I met at the Ricart Ford swap meet who had a bunch of assembly line NOS/takeoff parts. Nothing had service part stickers and most was bulk packed. (The few things that had labels were parts shipped from Ford Engineering) Weird stuff like a box of 50 or so door jamb switches, at least 200 inside door lock buttons. I also got 4 new take off 69 fastback decklids that I'm pretty sure came from Shelbys as they had the holes for the "MUSTANG" letters covered with a big strip of tape but they were painted...
Hope this helps, John