I think your stab is right on the money!
It seems 'production line' R3 Studebakers, (the nine '64 Avantis and Nels Bove's '64 Commander) did not have a fuel pressure regulator installed.....however....1964 Studebakers that had R3 engines installed at Paxton Products all seem to have that regulator! (An example of that is Nels Bove's '64 Daytona 'HRM' Special)
I guess the next question would be ...."Why did the folks at Paxton feel the need to add a fuel pressure regulator to an R3's fuel delivery system"? .......Possibly assumed high-speed usage?