These replacement CO2 Canisters are explicitly made for Hit-Air vests. 

We recommend always having at least one spare canister when travelling with your horse and vest. 

The Hit-Air 50cc canister is intended to fit Hit-Air Vest models: Vestige VH Model Small & Medium and Original SKV for Kids as well as MLV-CS, VHR SM, VHR MD, Vest MC3, Vest JV, HS-5, and MX-7.

Hit-Air CO2 Canisters come in three sizes: 48cc, 50cc, 60cc.

If you are unsure what size canister you need, unzip where the canister is housed on your vest, and you will find specific information about the vest you own, including the canister size. 

Always use Hit-Air canisters with Hit-Air vests. Generic canisters must not be used as you may experience technical compatibility issues that affect the vest's performance and proper function.