München-Mitte- Süd did have the color green, which stands for Oberbann 3.
This is according to: "Aufbau, Gliederung und Anschriften der HJ" from January
20, 1934. This is stated at page 24 for Oberbann München. Page 10 says: Die
Oberbanne werdem durchlaufend numeriert. Sie tragen zur Kenntlichmachung
besondere Farben. Maybe "L" was the exception and their Bann was not divided into Unterbanne.
Or the strap is from a later date. Have no time to check this. Maybe Garry can
jump in to this!