This looks like a rather well done conversion of a standard large American saloon. With that long rear overhang and the trailer weight on the back end, I imagine the steering was nice and light! Presumably not all ten of these firemen were on duty at one time; I believe the NFS worked a 48 on/24 off shift system, so the duty crew was presumably around six. Not too much room for any more!
The NFS was not a full time occupation for my father, who during the day worked at the SEBRO aircraft repair factory in Cambridge so would have been on fire duty evenings after the day job. I think must of the others had day jobs so the brigade would be as we now call retained.
Added by on 24 August 2023.
NFS so photo post August 1941 the black background to the number suggests the vehicle may have been relocated
Added by Rob Johnson on 23 August 2023.