for the gear setup, could the pulley have been a later addition? The Stallard references to the double pulley are from the mid-1950s.
1948 would have had a rod front derailleur, but most likely none at all.
There were later period rear derailleurs which used a tensioning cable as well as the gear changing cable.
One example is the
Huret Louison Bobet which the Dancing Chain dates as 1953
another is the
Simplex Juy 51, which dates from 1951, or the even later
Simplex 543
Does the frame have the twin cable eyes on the chainstay?
If it does not, then the second cable may have been for a, again later, bar end mounted gear lever (Cyclo or Campag) to operate the cable operated front derailleur.
The 1948 Stallard catalogue lists the gears as Simplex model 46, which I assume was the
Simplex Champion du Monde 46 in 3 speed variation. This design carried over from before the war. I have one in my spares box.
The production records held may confirm that, as well as the original colour.
Mercian have other colours.