Help Determining Age and Tonnage of AC

Hello,
Have old GE unit likely original to the home. House was built in 1978. I am trying to determine the age and tonnage of this system. The current coding conventions that I can find online do not apply to this system. Perhaps someone who has access to Preston Guide or a significant amount of experience can chime in.

Model # BTB936A100B2
S/N 250542922 (thinking this is 22nd week of 1979 but maybe not)

Any help would be appreciated.