The F-100, the F-4 (starting with the E and most later models) used leading edge slats. BVM employs leading edge slats on their F-100. They are very slick in operation although they are somewhat of a pain to do. (Just finished them on a D model) On his model they slide on some carbon fiber rails that slide up into a phenolic socket machined out of G-10. You can see some pictures of them on BVM's website. www.bvmjets.com look under the F-100 kit details.

If I am not mistaken they actually make the wing aerodynamically thicker.

DR