The oldest I think I've seen came out a couple years before I started playing." The Dragon Crown" in 1979 by Judges Guild was one of the first modules I bought, and it had crits (and fumbles iirc):In my experience (I'm from Spain) Crits were one of the first house rules most groups created in the AD&D era. These and fumbles, but fumbles were not so popular.
For the second roll after your natural 20...
1-14=like a normal roll of 20 (not a guaranteed hit)
15=max damage (15+ are guaranteed hits)
16=damage roll x 2
17=max damage x 2
18=damage roll x attackers level/HD
19=max damage x attackers level/HD
20=Instant Death