Please allow me to add to the mystery. The seat stays on my Limongi track frame wrap over almost exactly like those on the "Mystery" lightweight. But they don't meet. There's a 3/8" gap at the points. The underlying lugs appear nearly identical to those on the "Mystery" as best as I can tell with the seat post lever in the photo blocking the lower part. The Limongi chain stay where they attach to the track ends are "swallow tail." The seat stay bottom ends are not. They're chopped at a 45-degree angle parallel to the axle slot. Remember, Pepe Limongi was French.
Mark Fulton Redwood City California
PS: Super Vitus forks had swallow tails where the legs attached to the ends. Some did anyway. I don't know what that has to do with anything. Just sayin' as Chuck would say.