Originally Posted by
T-Mar
In the early 1950s Campagnolo introduced the low end Sport rear derailleur with a single pulley cage. In order to assist with chain tension, the top mounting pivot incorporated a spring which was anchored via a small retention hole incorporated into the dropout hanger or adapter claw. This derailleur was produced in several variants and during their lifespan Campagnolo's dropouts incorporated a spring retention hole to accommodate them.
Thank you [MENTION=20650]T-Mar[/MENTION] for the longer explanation! I've been researching Campagnolo quite a bit, but this was new to me. But it's probably because I've been excluding some of the low en stuff. Didn't know that some parts differed that much. Strange design, the component might be cheaper to manufacture but the dropout is more labor intensive and required for the RD to function.