Hello, John:
Berto's book says about the Constrictor on ebay:
"The 1947 Conloy Osgear rear derailleur used a parallelogram with with only one pulley. Its total capacity was about six teeth."
There's a drawing of another Constrictor on the same page, but it appears to be a rather different mechanism. Its caption reads, "This 1949 Constrictor rear derailleur used an ingenious planetary mechanism to maintain parallelism, but it still had only one pulley."
Berto's style is completely artless, but "Dancing Chain" is nonetheless invaluable.
Kris Green
Olympia, Washington State USA