Inflatable vest buckle extensions provide up to seven inches of expansion for both the chest and the waist buckles. This is enough room the same vest can be used over a another vest like the Casco MTR, be worn over a coat if it barely fits without one and allows use by folks of slightly different size. This also allows a shorter backed rider the option to get a smaller vest and just add extensions. There are two different sets of buckle extensions. Get a set for more adjustability.
The Standard set has two extensions of the same size for the Hit-Air SV2, LV, and the MLV (The Pro Vest).
The Advantage set has two extensions. The wider strap and buckle connect to the waist of the Hit-Air Advantage vests. The Advatage vest has a shorter waist strap with less adjustability than the other Hit-Air vests. The waist extension may be necessary for proper fit.
There is not a buckle extension set for the youth vest. At least not yet, sorry.