Unfinished Tales says it was a swan wing on the shield.Are you sure you mean sigil here? I mean... we know Turgons and Tuors sigils and neither of both has swan wings..
But the house of the wing in gondolin later has the emblem of the swan wing and they have winged helmets... we know the shield turgon left at nevrast bore a swan wing device... i don't think that there was before a housecof the swan but that Tuor newly founded that house at gondolin and that the members of his house either came from former members of other houses or formerly unalligned citizens.
I'm sure that his house emblems and helmets were inspired by the helm and shield turgon left at nevrast
Where turgon got the ideas for this from? I don't know... i guess i like the idea that they were either directly demanded by ulmo as a quasi-religious gesture or otherwise were part of turgons dreams/vision...
or maybe the swan symbol originally was a symbol from the iconography of the Falathrim who had moved to Vinyamar?
Question would be: is this the point where the entire swan-wing tradition which we will find the next thousands of years ,all through from the edain to the numenoreans to the royal guard of gondor and the swan-knights of dol amroth ,started or has it been there before as a sinda-tradition?