I think that everybody was baffled the first time he looked at the different pin-out of 78 and 79 series regulators,
but in fact it is the same:pin 1 is the most positive (input for 78, ground for 79)
pin 2 is the most negative (ground for 78, input for 79)
pin 3 is the intermediate (output for both).
In effect, sometime if you solder the wrong one, it will not be destroyed...
Cannot find the same logic in LM317 / 337, but I believe it is due to the pass transistor being connected to the metal tab for better heat spread.