Hmmmm! Certainly a lack of attention to detail on my behalf so, Simon, thanks for pointing out that the two 61XX tanks are different locos. Indeed they are. I also agree that the number plate could be interpreted as 613X rather than 615X. Also just wondering why you've ruled out 6136 as a possibility (I assume your description of it as 6163 is a typo). It was in the Didcot/Oxford area until 1965 so would not have been unheard of at Southall. Thanks for proposing 6135 as a candidate too but we can discount that - see below.
Dave, your photo rules out 6159. Tim himself has a photo of 6135 at Old Oak in 1964 with smokebox door lamp iron but it still carries the smokebox door numberplate, as does your photo. (Proving that I should have checked for other photographic evidence!) I agree that 6158 is a most unlikely candidate for the reason you describe.
Col - your post sent me down the rabbit hole of looking for other photos of 61XX locos at Southall. Here's one of 6156 with lamp iron on the smokebox - in 1965 but to my mind a possible candidate for the second of the photos. It's also a photo I took!
The first of the two photos in post #1034 carries an 81D shed plate. In 1964, and in the 613X and 615X series there were only two locos of this class allocated to Reading, 6131 and 6134. I don't have a photo of 6131 but I have the other one. And it's mine again! Southall 1965 and both these were taken on the same day, 5th December. No smokebox number plate and the lamp iron on the door.
If you point out that this is all circumstantial I agree. However, to my mind both the above are possible or even likely candidates - unless you know different! In any event please accept my grateful thanks for, at the very least, proving what the locos in these two photos are not. Incidentally, Simon, that photo of 6135 at Old Oak in 1964 put up by Dave appears to be unlined black rather than unlined green, notwithstanding the difficulty of interpreting real colours when images are printed. I remember a certain number of these locos in black so it's entirely possible.
I have more Southall photos lined up but have expended more time than I intended on this thread today so will stop here.
Brian