Hi sv,
The US Box springs picture and pricing is currently their regular wooden slatted foundation (not the metal one) which has gaps of between the slats of under 3" and a quilted cover.
They will have both listed in the next day or two … just to make sure you know what you are looking at.
Also the metal version described in post #15 here comes in regular twin, queen, and Cal King sizes only (no twin XL or Eastern King).
I don’t own one so I can’t comment about it from personal experience but based on my conversations, the metal frame will probably be the last foundation someone will ever have to buy (it’s very strong).
Phoenix