Service replacement Muncie, as per
http://www.camaros.org/trans.shtml#service
C=Chevrolet
T=Transmission
39099=unique unit sequence number
Muncie transmission plant was assigned the sequence range 35000 to 44999
Perhaps the 'A' denotes the full range allocated was used up, and they reset the sequence numbers, with the addition of a letter prefix, as happened with engine blocks.