Note that nowadays there are two styles of ‘double-throw’ XOP switches. See
http://ecatalog.squared.com/catalog/173/html/sections/03/17303014.html A newer style has
two sets of movable blades and two sets of fixed jaws. Older is
one set of movable blades and two sets of fixed jaws. (Note upper vs. lower photos.)
[I believe Cutler-Hammer has a similar variation.]