The H Model is a longer narrower design. Small and Medium take 50cc cartridges, Large takes a 60cc cartridgeThe H Model is a longer narrower design. Small and Medium take 50cc cartridges, Large takes a 60cc cartridgeChildren’s Vest, suited for those kids that aren’t quite big enough for the H model Small.A more triangular designed vest that fits people who aren't suited to the shape of the H Model vest.A more triangular designed vest that fits people who aren't suited to the shape of the H Model vest.