12mm refers to the spacing between the conductors.
This connector style was designed as a home television antenna
connector, in Germany. However, the contact area is
sufficient to carry a great deal of current. Between
that and ready availability, I suppose we can understand why Widder
decided to use it as their connection for electric clothing.
Widder now makes gear for Harley Davidson, so Harley electric
clothing also uses this connector. BMW uses it as well, for
some of their gear. From my limited experience, it appears
Widder has standardized on the female connector as the battery
side, and as you look down at the screw terminals with the sockets
pointed ahead, the hot wire is to the right.