So did the Morris Commercial J-Type 10cwt Van have an interior light factory-fitted from new? Kim says yes, it was made by Wilmot Breeden and sat on the driver’s side sliding door cover just behind the drivers head. Kim says that you may find the original two small fixing holes about three quarters of the way up toward the front of the panel.
The light is shown in a Morris JB parts manual he has, and Kim has seen the same light fitting (although broken) in an old Austin truck (3 tonner?) so it could be a standard BMC fitting.
And then there is this, from a 30 January 1952 supplement to the 'Motor Trader', on page vii. This is courtesy of Roly via the MCJTV group:
This shows that the interior light was a Wilmot Breeden 7/2205 light fitting with a 12V 6W festoon type bulb.