I know this thread has been going a long time, but it could be important.
I’m not sure that this was a NEW weapon at all, more like a very old one. Which may explain why no obvious repeats.
During and after WW2, various British and allied anti-tank guns had “littlejohn” adaptors developed for them, which was basically a squeeze-bore arrangement. (Concurrent with Nazi developments.) Littlejohns were developed for a lot of things, including .55” Boys AT rifle, but I think the only ones that saw serious testing and even service were those for the 2 pounder AT gun and 40mm “S” gun. The 2 pounder version WAS used, in Burma. Weapons would have been shipped through Iraq or nearby. In Europe, commonwealth troops just used bigger guns, but not really an option in Burma given the terrain and huge distances being fought over. Jap armour was less of a challenge than getting any sort of heavy weapon to the front!
Littlejohn adaptor definitely supplied, for field trials, to airforce units in North Africa with 40mm Vickers “S” guns on Hurricanes, these , too, went to Burma via Iraq after German defeat in North Africa, as Hurricane not FW190 proof! (Nothing much was, prior to Tempest.)
Mystery weapon could have been a 2 pounder or cobbled-up “S” gun with a few (possibly even just one) rounds of ammo. It wouldn’t totally surprise me if Iranians were interested in copying squeeze-bore technology, as guns easier for them to make than missiles. They’ve made a credible copy of 76mm Naval gun recently: a littlejohn adaptor on that might make holes in almost anything. 76mm naval gun really derived from 17 pounder AT, via several decades of wandering development.
Littlejohn not very successful in forties, as barrel steel and propellents not really good enough. And the seventeen pounder was available, which could kill Tigers, the six-pounder could kill everything else and remained in UK and US service till the sixties. But it was a way of getting something the size of a two-pounder, to kill something approaching a Tiger in strength.
Challenger has more expendable junk stowed on the OUTSIDE rear of the turret than Abrhams, which may account for lower mishap rate for the type. The main threat to Challengers, so far, has been other Challengers commanded by Scotsmen. The main threat to Alvis Spartan family vehicles, used by NO country hostile to America, has been the USAF. Apparently, USAF thinks it’s somehow improper to teach pilots basic vehicle recognition, but they might have more allies, for longer, if they did.