Yes, Mugen kit = round hole with exposed gas cap
I've got a small selection of 2.5" rear springs I'd like to try out, but Tein springs are different-enough in OD/ID that it's not a great idea in my head to simply stick 2.5" (ID) springs under the Tein (gold colored) hats. The only Tein springs I've got are the 5kg/mm which are on the stiff end of what the shocks are valved for...the ride quality with these is a bit like driving an unloaded dump truck downtown.
So, with access to some old manual machine tools, I carved out a pair of spring hats that retain the Tein exterior to fit the rubber inserts in the body and altered the spring seat area to better match the 2.5" springs.