There was a sleeve that is sometimes seen fitted on Valentine VIII,IX and X to protect the counterweight mounting threads, it must be emphasised that the Mk5 counterweight is a substancial lump. Some 6pdr barrels were produced as a universal component and could be mounted on the two types of tank mount, hand wheel or free elevation or the towed weapon in which case a single barrelled muzzle brake was fitted. There is also some evidence of a muzzle thread protector on some 6pdr Mk3 barrels for the same reason. Sometimes what looks like a counterweight is nothing more than a weather sleeve or what's left of one.
The lend lease tank book which is in Russian and Steve Zaloga and James Grandsen book Soviet Tanks (1984)both have good coverage of Lend Lease tanks.
With regard to Churchill on Lend Lease there seem to be a high percentage of Mk4s of various styles including some rework/rebuilds (for example T31620R), the Mk3s shown have Mk3 guns, It is interesting to see that Chris has found evidence of MK2 being supplied, there has been reference in the past of Mk2s used as support and control tanks for SU152 SPGs.
There were also samples of Mk7 Crocodiles supplied one still exists at Kubinka. According to the editor of the Russian battlefield web site the Soviets tended only to refer to Churchills as Churchill Infantry tank MkIV and never differentiated Marks such as Mk2,3,4 or even Mk7, although the Mk7 were only sent for tests.