There are several claimants for todays " Syl "..
Laure Haille ( spelling may be different ), who worked briefly for A. Murray's in the late 40s and 50s,put the 1st one together at that time.
There have been additions over the years , from several sources . And, there are steps in the Syl, that were co-opted from other dances .
If you want a detailed history, go to Dance Forums and the Swing section. There is a biographer (Steve Pastor ) who has written numerous articles, on Swing in general.