Wayne Smith built the Sleng Teng rhythm himself so maybe the version Promotion are playing is one of the first dub-plates of it. The fact that he's also in the dance doing it live may mean he could have taken the dub-plate to the dance.... Just a theory, but you cant argue with the facts that this dance vs Gemini was definately an earlier date than the Jammys, Scorpio clash that has been posted. Also they say they are the only sound playing it.
If you also listen to Youth Promotion at the House of Stars 84 (there is a released LP of this called George Phang presents Strictly Livestock on Greensleeves records) You hear Wayne Smith sing a prototype version of Sleng Teng, not on the Sleng Teng rhythm, but more linkage between Wayne Smith and Youth Promotion...
I think there may have been 2 clashes between Jammys and Scorpio not far apart so the one where Jammys debuts Sleng Teng could have been at an earlier date, but tapes from the other clash have not surfaced YET! so maybe we will find a definative answer one day.
As for Coxsone playing it in January 85, again it is documented on tape so that is fact. Coxsone have strong links with Sugar Minott, so maybe they got Sleng Teng from him... Again Theory, but a possible reason..
A very interesting subject and one I'm sure we have not heard the last of!
PEACE