I picked this up from a current eBay auction of a broken second generation Rally:
To quote an online review, this apparently is a known defect for this otherwise great part, modified quotation follows: The Rally 3450 derailleur seems to have been introduced in late 1974, about the same time that the prototype Super Record was made. The upper spring-loaded Shimano-style upper body is made from pot-metal with a thin chrome plating. The early Rally's also had a thinner "neck" below the upper spring...the derailleur broke here due to the porous pot-metal, and the "neck" was made thicker in roughly mid-1975.
I've been trying to buy one of these for months, but now I'm not sure I want one.