In the video you suggested, https://www.youtube.com/watch?v=QTak2_MrgQs that car appears to have almost no under coating on the floors and wheel wells.
Just to help. Typically we refer to the thick tar and asbestos material that was applied after the car was delivered to the dealership as "undercoating" and the stuff applied at the factor as "sound deadener" to keep them separate in discussions
Are you referring to sound deadener in the rear wheel wells or front. Ford applied it to the rear wheel wells before body color was applied over it and I doubt that missing this step would pass any of the hundreds of inspectors on the line. As far as sound deadener being applied to the floor area that depended on the model of Mustang and the options in most cases
Also no seam sealer on the seat belt mounts. was it removed or is that typical of production?
Likely removed, cracked and fell off or cleaned away at some point
If you look at my second picture, the undercoating on this car spreads from just past the transmission tunnel to the rocker panels on both sides and in the wheel wells. When it was sprayed, it appears the worker was standing under where the right-side muffler sits as that seems the be the spray pattern direction. It is also sprayed lightly in the tunnel above the transmission closer to the firewall where the firewall and floor pan come together. I don't think it was dealer applied because there is no way a dealer could have sprayed it there without removing the transmission.
Didn't watch the video but if there is spray on thee muffler then that is very likely and indication that someone on the line caught an issue and it was touched up or that is undercoating applied later. Mufflers as we understand it were installed after the factory floor sound deadener. For above the transmission it was installed before thew sound deadener so the workers didn't want any on the transmission or linkage to screw operations later so typically the factory applications started to the sides and behind at the plant.
Not the typical location of the spray so likely undercoating. Easy to get to that area with the long wands used to apply the nasty stuff in the 70's
Though different year, same plant some of the information in the article below may be of help. Found it using the search and the words "69 undercarriage" and my screen name as author
https://www.saacforum.com/index.php?topic=929.msg6979#msg6979