447 = transmission build day 447 = March 22nd, 1969 (54 years ago yesterday). 447 - 366 (number of days in 1968 (it was a Leap Year)) = 81st day of 1969 = March 22. So this transmission would have been installed in a '69 GS that was built sometime in very late March '69 or after.
Just above the pan rail on the driver side there should be a partial VIN. Here is an example from a '72 model 4= Buick, 2 = 1972, H = Flint, 109111 = last 6 of the VIN.
do you know what the 71 year would have been my car was dated 05c?would it have been built in may of 70 or 71?