Found this on another board:
The different sized spouts are for different CO2 per vol levels. The 12.5mm spout is for 2.9+, 12mm = 2.6-2.9, 11.5mm = 2.3-2.5, and the 10mm is for 2.2 and under. They are used to further tweak the pour velocity.
Hope it helps answer your question.
"Chicks dig me, because I rarely wear underwear and when I do it's usually something unusual." John Winger, Stripes
On Deck: AHS Fat Tire, AHS Belgian White
Primary 1: AHS Grand Cru
Primary 2: AHS Boddington's
Secondary 1: Dead Head Draught
Secondary 2: Open
Kegged: AHS Fat Tire Clone / NB Irish Ale / Dead Head Draught / NB Honey Kolsch
Bottles/Mini Kegs: NB Bourbon Oak Porter / Yorkshire Ale / AHS British IPA / Double Chocolate Stout / NB Honey Kolsch