Looks like someone has been in there since the car was at the factory. Possibly a taillight panel replacement or some repair for some other unknown reason.
Here are some unrestored examples from late 70 Dearborn built Mustangs
As you can see comparing it to your car there was no sealant applied around the taillight panel on the sides and top
At the rear bumper bracket to floor edges a sprayed on (like around the wheel houses to trunk floor side) sealant was applied. very thick and with ripples from the application
Long the lower edge of the taillight panel to rear trunk the sealant was most often applied very neatly. Applied to the gap then wiped. On a few examples I've found where the guy applying the rear bumper bracket to floor sealant ran his applicator gun along the seams but this IMHO, at this time, this appears to more be the norm, just mentioning it
There is now an article in the library that covers these details as part of the undercarriage finishing artile you might find helpful also