RE: PICA Spitfire 1/5 Please Help
I read about the "U" shaped mounts in Harry Higley's book "Mostly Mounting". Below is a sketch of what they look like. Basically, the mounts take the place of the gear mounting rails, and offer quite a bit of rigidity (if that's a word) to the ribs they are glued to.
Hope that helps. I could include a photo from the book, if that's not against some rule.
Sam