Thanks for the prompt responses. Note that while I got the rarity part wrong, I did doubt these Vickers guns were actually LMGs (I put quotation marks around "light" when first introduced). Think I know how these MGs were deployed.
> The water jacket was retained and connections provided on the Mark 6 for an internal header tank for coolant water. >
Note in the first of the three photos taken of the members of 2nd Battalion, Argyll and Sutherland Highlanders, the crew member is leaping pretty nimbly from the rear of the armored car, handling the Vickers machine gun (Mark VI or VI*?) with ease. I conclude from that and the second photo that no water is in the gun's water jacket, which otherwise would have increased substantially the burden on the carrier. The other soldier in the first photo is consolidating the contents of a second water chest into the first one (on the ground). There is no drainage tube shown in the third photo, of the gun in place ready to fire, so the water chests shown were likely not
condensation cans. In regular Vickers infantry MGs, water vapor produced by the increasingly hot gun barrel within the water jacket---roughly 1.5 pints evaporated for each 1000 rounds fired---drained into and condensed within the drainage tube, after which it collected in the condensation can. Rather, the water from the chest shown in the photos was poured into the water jacket of the machine gun once the gun had been emplaced, and likely that's where that internal header tank came into play. Also, the probability was that only brief delaying fire was anticipated before the crew returned to their vehicle and moved on.
The first and third photos of the Lanchester Mark I armoured car deploying their Vickers MG are repeated here for convenience in viewing.
Again, does anyone have any details on the actions involving Lanchester or other types of British armoured cars in 1941-1942, particularly on the Malayan mainland?