According to my research this should be a 3 SUB but clearly is is 4 cars. These two images of 1504 look like a posed publicity shot. Could it be August 1945, when Eastleigh works were adding an additional coach to units 1496-1524 for the Eastern Section?

This shows the unit at a stand-still just before Lordship Lane station on the Crystal Palace (HL) branch.
It is not running wrong line but rather an ‘official’ photo where presumably the driver has walked from his cab at the front to the rear. Note the tail lamp. Bags of time, the weekend service was hourly. Or possibly there was another crew member already in the cab.
Photo taken from Cox’s Walk footbridge over the track.

Photo: M. Gaywood/Rail Archive Stephenson