The TIS kit was introduced on the Mk 20 but some of the components were present on the Mk 15.
Basically on a balanced piston regulator it includes a piston head bushing (which insulates the piston and controls/limits any ice development), a flexible rubber piston sleeve (which insulates the expose portion of the piston stem and also cracks off any ice that may form there), a teflon coated mainspring, and a composite piston with a mirror polished piston stem.
The MK 25 "AF" kit added the finned swivel cap. But since the AF kit became standard on all Mk 25's, there is in fact no Mk 25 "AF" even though many people call the standard Mk 25 the "Mk 25 AF".
If you upgrade to a new piston, it will come with the new bushing (dealer cost is about $21.00 I think). You could also add the finned swivel cap for about the same cost. The rest of the AF kit will be included in a MK 20/25 annual service kit.
