Total # of Occurrences: 24
First Occurrence: October 12, 1924 - Racine Legion @ Chicago Bears
Last Occurrence: November 16, 1997 - Philadelphia Eagles @ Baltimore Ravens (OT) - so 2 of the last 3 tie games involve the Eagles on the road against an AFC Central/North team
Occurrences after 1970 merger: 9 - also the most common score for overtime ties since 1974, including the first - Rams-Vikings on September 19, 1976. The tie helped the Vikings more, meaning they finished one game better than Los Angeles, allowing them to host the NFC Championship. Had the Rams won, they would have been the #1 seed, I think, since their conference record would have beaten out Dallas and Minnesota.
A 10-10 tie helped the Vikings two years later as their draw with the Packers on November 26, 1978, along with the rest of the games, put both teams @ 8-7-1. Minnesota won the other head-to-head game that year against Green Bay, so they won the tiebreaker and the NFC Central. Chester Marcol missed an OT field goal that day.