I attach three views of the tail end of the same Mk IV In the first two taken within a month of each other a thin transverse bar can be seen with a much thicker round tube (?) beneath it. There are other photos showing the same features. Thes last photo taken possibly a month or so later merely has the transverse bar. Again I have similar photos. Finally in photos taken later even the transverse bar has gone and there is just the normal Mk IV rear. Does anyone know what this feature is? Has it been seen on other Mk IVs?
My only two guesses are a] some sort of towing arrangement and b] The thicker tube is some sort of smoke generator; but I wouldn't bet the farm on either. All the photos with the thick tube were taken when the tank was involved with troop training.
Although I haven't been through every photo of a Mark IV's rear (!) that's available to me, the only Mark IV I can find that shows this feature is Britannia, the tank (or tanks) that toured the USA and Canada.
That being so, I wondered whether it's unique to Britannia. It also occured to me that alot of dignitaries would have wanted or been invited to enter Britannia, and the bar or bars would make for a handy step up to the rear door. It would otherwise have been quite an undignified entrance, either via the doors beneath the sponsons or clambering up the tracks at the rear to the roof and then through one of the roof hatches (Brittania having two, of course).
No evidence whatsoever behind this speculation, but it's the best answer I've got.
However it (she?) only appears to have had these features when involved in troop familiarisation and training. They would seem to have vanished by the time, for example, that President Wilson went for a ride in Britannia (possibly why he burnt his hand when climbing out the roof hatch) in practice I think that a revolutionary device known as a step ladder (mk XI) was used in many cases. I wondered if it were a specific feature (which is why I was a little elusive in my framing of the question).