Republic Aviation had 2 plants - the first one at Farmingdale NY, and a later second one at Evansville IN. The Farmingdale plant used an RE, while the Evansville plant used RA.
EDIT! Changed "The Farmingdale plant used an R, while the Evansville plant used RE." to "The Farmingdale plant used an RE, while the Evansville plant used RA."