I have also tried to find out about this mark, but unfortunately with poor results. I'm not fully convinced (yet) that it is Odessa's cipher. Its cipher very similar to chi but it is the written kappa (K) letter. The cipher chi is for Don and therefore we should maybe look at Astrakhan, but I haven't found anything there either.
With some fantasy the cipher could also be the written ni for Kiev, but...? Nothing found there...
Is there any marks behind the neck and/or below the chin e.g. dot(s), dash or anything?