Back to the Future.......
Back when dinosaurs roamed the earth, there was a regional/conference LOI date, usually in Dec or Jan, with a National LOI date in the spring. Just because a kid signed in Dec did not necessarily bind him to that school the next fall.
All that changed sometime in the '80s or so, probably somewhere within a decade or so of the 85 Rule. BTW, the 105, then 95, now 85 Rule came about as university presidents tried to get control over the perceived runaway costs of schollies in that era and give the general public the false notion they were somehow in charge of their athletic programs. Travel squad sizes, walk-ons, etc., were also part of that economic flail. At one point GT was rumored to have 165 (yes, one-six-five) S-As on full football schollies, so you can imagine how it felt to be a walk-on who was at least playing part-time as opposed to full-ride guys who never dressed on Saturdays.
Just as a point of historical reference, former GT QB Charlie Dudish didn't sign his LOI until June 1968 at a Saturday AM press Conference at the old GTAA, just after the TV special on his recruitment aired. Six to eight weeks later he showed up on campus for Jerry Glanville's Freshmen Boot Camp.... uh, Fall Practice.