1-19: Quarterbacks and specialists
20-49: Running backs and defensive backs
50-59: Centers and linebackers
60-79: Defensive linemen and offensive linemen other than centers
80-89: Wide receivers and tight ends

Numbers 0, 00, and 90-99 are no longer allowed to be issued, even though these numbers were rarely issued anyway. According to the American Football Database, numbers 90-99 would be allowed again in 1984 for defensive linemen and linebackers in addition to the above-mentioned numbers.

Other NFL rule revisions included the following: Defensive players cannot jump or stand on a teammate while trying to block a kick.
The clock is to start at the snap following a change of possession. Previously, the clock would start on a change of possession when the referee spotted the ball ready for play.

If the offensive team commits a foul and follows it up with a change of possession, the period can be extended by one play by the other team.
If the receiving team commits a foul after the ball is kicked, possession will be presumed to have changed; the receiving team keeps the ball.

Of course, this had some tweaks along the way with expanding the numbers for skill players in 2020 and the reinstating of the number 0 in 2022.