The New Yorks Show Lightning was a different bike to the Earls Court Lightning.Simon, was that the same bike they used for 1949 Earls Court?
I've just looked at a photo of the Earls Court bike taken at the show and that was fitted with a rear head and the records say TT10 1-5/32" size carbs which appear to be flange mounted.
The photo of the Vic Proctor Lightning when new and leaning against its packing crate also shows a rear head and that also had TT10 1-5/32" carbs. Cannot tell though if they are flange mounted or not.
So don't assume all Lightnings had two front heads.