The Royal Naval Air Service's Royal Naval Armoured Car Division markings are read: Squadron . Section . Car Number
"1.B.4" would be 1 Squadron, 2nd Section, Car #4.
The lettering on the armoured cars is often depicted today as begin either black or white, but a 1915 painting by J. Lavery entitled "Armoured Car Squadron" shows a squadron of dark "battleship grey" armoured cars with "RNAS" lettering on their sides in blue.
I suspect the variation seen in the old B&W photos is due to the orthochromatic film in wide use at the time that was over-sensitized to blue, making it appear very light in some photos, while with less blue-sensitive films it looks dark.
On a related note, the cars were individually named much like ships. The first letter of the car's name matched the Squadron number, so all of the cars in 1 Squadron had names beginning with "A", those in 2 Squadron began with "B" and so on.
