I think my vair posi was a 3.89. I remember changing the plates. I do not remember the letters, but I do have an orig GM Corvair parts book for all the yrs. I also made ( converted) a vair trany into a close ratio box with Corvet Eng help. They provided the part # for 1st gear that was used in the Saganaw Box. I think the Impala used that gear? Dick in MD Ex Yenko#99