Tail Shaft Leak - was "Is this gasket available? A904 Extension Housing Snap Ring Cover"

The outside boot you see is indeed a rubber dust boot. However, there is a conventional seal made to that that presses into the rear of the transmission tail shaft housing. The rubber boot on the outside seals the outside of the slip yoke from dust and dirt entering into the main seal area and possibly damaging the main seal that seals the yoke and keeps the fluid inside the transmission. Not all those tail shaft seals have the boot, but I prefer them. Kinda gives another layer of protection.