The 123-123-12 rhythm appears in Middle Eastern, Balkan, and African music; I would more than suspect that its occurrence in contra dance music has come mainly via the African route, both via the slave influence in Appalachian music and via the hippy/funky influence in modern contra.
The klezmer/Romanian 123-123-12 has a different inflection to it - a different articulation - the late great Balkan dance/int'l folk dance teacher Dick Crum called it a "Get your Papers Here" rhythm - more of a 2;1,2;1,2 articulation than a 3;3;2 articulation.
...Unless the rhythm you're thinking of is the rock-n-roll boom-chuckboom-boomchuck - in which case we're back to the African influence...
- Yaron