Yep, those are the usual 4" Smoke dischargers. Pretty much standard on British AFVs once they were over the "OMG the germans are invading" hump.
My Humber will have two. One of the partners in the restoration picked up a bunch from the old Navy Arm's C&R dealer. They're essentially a 4" pipe with a threaded base and a cut down SMLE action and barrel on the back end. On the Humber LRCs and Armoured Cars they're fitted firing through the front armor. On most other vehicles they're fitted to the side of the turret. Firing is either by a crewman reaching up and pulling the SMLE action's trigger OR if external, pulling on a trigger that's attached via a bicycle brake type cable fitting that runs outside the turret armor to the trigger guard of the SMLE action. They're fired from inside but re-loaded from outside.
The 4" smoke bomb is a pretty good sized smoke generator and makes a very large volume of smoke.
There was another kind of action that had a similar setup, was 4" but was a pressed sheet steel affair and wasn't as pretty as the SMLE action was.
Shermans in British service had a breech loading 2" Mortar bomb thrower. Basically it'd take the British 2" Mortar Smoke bombs up a break action type affair and throw them. It couldn't be aimed for elevation so was largely useless for throwing regular 2" mortar bombs.
There's a video on Youtube showing the 4" SMoke discharger being fried from a MK3 Humber AC. It also shows off the Humber Armoured Car and british Recce doctrine quite nicely.
http://www.youtube.com/watch?v=qJC2dWdCGGc
Note, the 15mm Besa and 8mm Besa are both seen and heard firing in this old film.