I think you're describing AD&D 2nd ed (2nd ed AD&D PHB p 13).While things have changed, in Ad&d, you rolled your ability scores (Method 1) 3d6, in order.
In the original AD&D DMG, the following options are given (p 11):
Method I is 4d6, drop the lowest, six times and arrange in order. (This is 2nd ed Method V.)
Method II is 3d6, rolled 12 times, keep the best 6 and arrange in order. (This is 2nd ed Method IV.)
Method III is 3d6 rolled 6 times for each ability in order, keeping the best in each set of 6 rolls. (Method II in 2nd ed is like this, but with the best of 2 rather than best of 6.)
Method IV is what you describe, done 12 times, with the player than choosing which set of ability scores they want.