Hmm. I also found this. Looks like it depends on where the car was made. R3 is the engine type. http://www.theavanti.net/r3.html
Studebaker Avanti 1964 Production R3 Profile
Serial Number
Original Locations
Paint Color
Transmission
R5089
Downers Grove, Illinois
turquoise
4-speed
R5237
Seattle, Washington
black
4-speed
R5394
Kingston, Pennsylvania
white
4-speed
R5532
Vauxhall, New Jersey
turquoise
4-speed
R5546
Fort Wayne, Indiana
gold
4-speed
R5593
Yuma, Arizona
gray
automatic
R5625
Toledo, Ohio
black
automatic
R5642
Moline, Illinois
white
automatic
R5643*
Jackson, Mississippi
white
automatic