Here's the thing: I have no real reason not to believe that the three/four speed trigger shifter isn't original. The catalog indicates that S-A bits and pieces could be specified as the buyer wanted, i.e., you could get whatever configuration that was available at the time and the trigger shifter seems to be the more likely candidate. However, there is some banded scarring along the top tube, right where a quadrant shifter would seem to have been mounted. And here is a 1952 Elswick that was found (believe it or not) as an NOS, partially assembled kit in Ireland - and it has the bar mounted shifter:



Bottom line is that the jury is out for the moment, especially since the trigger shifter seems to function fine and definitely is period correct. If my friend doesn't sell me the quadrant shifter, the point will also be moot anyway!