Would agree with Pete that like many the cleaning process has removed with it multiple or single layers of paint (heavy or lighter overspray patterns) during the process., Also I think you've figured out or have not cleaned the inside of the shock tower where someone rattle canned that surface. To others reading that inner fender marking is not something all 69 Shelbys came with so don't copy it blindly
As you may have already discovered the patterns are the same as any other Mustang built at Dearborn that year except they skipped the sound deadener application since there were not fenders installed at the plant
The base was a red oxide colored epoxy based primer sealer followed by an extension forward of the body color on the outside of the rear inner fender panel sometimes all the way into the shock tower and in some other examples the front edge of the shock tower. Different side were often different. Then when the engine compartment was painted along with the radiator support the black could be introduced slightly when the face of the radiator support was painted - producing a shadow and some overspray of black on the forward inner fender panels. In other examples the painter applied the black directly to that front inner panel extending rearward, sometimes to the forward edge (or about there in places) of the shock tower. Final look is over three colors/finishes - From the rear - body color - red oxide then black.
Of course this would be over the earlier applied messy seam sealer at the rear
Basic possible look from the article
A few pictures until the link is fixed. Ignore, please any of the sound deadener in the Mustangs pictures they still help understand some of the process and possible spray patterns. Edges of paint can be jagged or sharp since during cleaning their finer oversprays have also been removed
Have plenty more just in a hurry to go pick up an order but wanted to post to help
