Good question. I do know that lettered sweatshirts, letterman jackets, and sweaters were worn in (and probably even before) the 1960s. They may've only worn these for certain types of events and it may also vary by campus or type of GLO.
It made me think back to when this old man (probably about 65-70 years old) came on campus during homecoming with his lettered sweatshirt on that was clearly old. I think it was Sigma Chi. A couple of students laughed at him. My Sorors and I were like
because when WE see older Greeks (and not just BGLOers) with letters on they get the utmost love and respect from us.