Clearly, it was a modern mock-up of a Dicke Berta (or similar). I've attached the full-size image - it's not very accurate (among various problems, the radgurtels are perfunctory and what are all those rivets doing covering the breech?) but it's always fun to see something from the era on film, and makes a change from the entertainment industry's obsession with the Second World War.
Anyway, it transpires it's from a German mini-series:
Wow, that is one mean looking mock-up. Weird riveted over breech block as you say, but none the less...
Totally agree with you on TV's fixation with WWII. OK, with seminal programing like 'The World at War', BUT most of the output I've noticed on the current schedules seems to be trying to squeeze every last piece of sleaze, sensation, attrosity, perversity and conspiracy out of Hitler's life & conduct as well as that of his Generals, key members of the NSDAP & SS, which is frankly tired tawdry gossip mongering rather than informative researched historical broadcasting.
The ocassional worthwhile WWI doc does pop up though - I recall the excavation of a WWI British tank, the unforgettable Livens Large Gallery Flame Projector, a dig at the Vampyr trench system. So, mostly 'Time Team' then. Although I think Kenneth Branagh narrated a WWI series, not seen it though.
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"You there on the port!". "S'gin actually, but thanks for noticing [hic]".