Sears GT 19.9 tractor. Years built?

I have an old 70's era Sears GT tractor with a 19.9 Onan twin engine on it. I've had the tractor since 2005, when my neighbor wanted to get rid of it, so i took it and restored her to her former glory. It runs great. However, i am not sure what year the tractor was built. I think it's a 1977, but i am not sure. Can anyone help me? Thanks.

i have one of these i bought several yrs ago used, i thk it is an 79 or 80 i have the book and the add from sears cat. when i get back home will try to find info