That would be the Motordyne Thermal plenum spacer, sounds like an added expense without much gain from what I've picked up on here, try searching for "Motordyne thermal plenum spacer" on here, I think "Abbey M/S" has commented on them before.
EDIT: Quote from Mark@Abbey: "I wouldn,t bother with thermal spacer kits never seen any gain from turning the water off through the throttle body."