Legends in their own minds

We now return to exegesis of the Man Utd love-in that appeared in Thursday's Washington Post. The following claim is made:

United won the Premier League title seven times, the FA Cup three times and the European Champions Cup. They won all three crowns in 1999, known as a prestigious "treble" -- which had never been done.

Notice the omission of the word 'once' after the Champions Cup reference -- a word that usually attaches, for instance, to descriptions of the Atlanta Braves' achievements over the last 10 years. Furthermore, the validity of this claim depends on what you mean by 'never.' It had never been done in England. But just across the border in Scotland (and therefore in the United Kingdom), Glasgow Celtic did not just a treble, but a grand slam in their legendary 1966-67 season. They entered four competitions: their domestic league, Scottish league and FA Cups (knockout competitions for all league clubs, and all clubs, respectively) and of course the European Champions Cup, the same elite competition won by Real Madrid seven more times than "the world's winningest club.". And Celtic won them all. A tribute site is here, and a description of their European campaign is here; check out the history of a truly trailblazing season.