I think Adam has probably hit upon the right direction. There is record of 44531 at Shoeburyness shed and therefore it worked on the LTS. This line was uniquely equipped with Hudd ATC and I believe it became mandatory for locos to be so equipped so I think the fitting in front of the cab is an auxiliary reservoir for that system. I can't find a picture of any other vacuum 4F so fitted which is possibly an additional pointer. I should stress this is total surmise on my part.
Regards
Martin
Addenda: I've had a look at LMS Journal No 12 which has an article on the development of the Hudd ATC. It states that on 1/12/47 the introductory date for the system on the LTS there were 183 locomotives fitted, including "various 3F 0-6-0Ts and 4F 0-6-0s", and certainly at some point 2 cyl 2-6-4Ts at Kentish Town although no direct mention of 4Fs at that depot. The article also states " it was forbidden in LMS and early BR days to run unfitted engines on the line as fogsignalmen had been withdrawn". This somewhat reinforces my point made above and the appearance of 44531 at the far end of the LTS means it was ATC fitted and the the cylinder in front of the cab is the main vacuum storage cylinder (for the ATC). The locos themselves being I think steam braked wouldn't have had a vacuum reservoir for the brakes.