My frame is a De Rosa Professional from early 1980's.
The chain stay tire clearance area near the shell is where the diamond shape is. De Rosa had a special pressing tool at the time to reinforce the chain stay behind the bottom bracket, and he was the first builder to use micro-fusion bottom bracket shell, lugs, and fork crown. Also E.Merckx used the same UD shell and lugs for his frames, minus the diamond shaped stays which was UD trademark.
The chain stay tire clearance area near the shell is where the diamond shape is. De Rosa had a special pressing tool at the time to reinforce the chain stay behind the bottom bracket, and he was the first builder to use micro-fusion bottom bracket shell, lugs, and fork crown. Also E.Merckx used the same UD shell and lugs for his frames, minus the diamond shaped stays which was UD trademark.