The Polish version. It's development was halted was halted in the late thirties for some reason or another, but it was rumoured to have been the only CW used in W. Europe in WWII. There were accusations of use of mustard when the Germans first invaded Poland, if I recall correctly it was referred to as the Jaslow Bridge incident. The Germans used it as a propaganda point, claiming that Poland was provided mustard by the British, which they strongly denied. To this day there seems to be a lot of debate, I heard some Poles state that the entire incident was German propaganda, others that Poland was up against the wall, and justified in using whatever they had to defend themselves.
Two versions of the mine were built, the CW and an incendiary variant. As far as I know only two remain, a cutaway of the CW and a still full incendiary (at least 10 years ago it was still there). Both were located at their birthplace, the Polish NBC center, someplace near Warsaw - I never learned my way around so don't remember the exact location.
I took photos of both for the OCW handbook, I can try to look up some pictures this weekend.