Thanks to Ross for sending me this!
Commentary by Redditor paulkempf:
Looks like a post-war mod, ISU-152K (additional fuel tanks on the back, track guards and storage boxes give it away, but no extra armour on top of the manlet, as far as I can see, so it’s not a ISU-152M).
Below is a rough translation of the blog post accompanying the video.
It was found in Holinka, Chernihivs’ka oblast, Ukraine, in 2003, but wasn’t recovered until 2012. Apparently the village officials got their hands on it some time after the war (it had to be well after 1956, when the ISU modification programme started) and were going to turn it into a memorial.
They never got around to it, used it as a tractor for a while and then just left it there to rot. It was looted and all the copper and aluminium parts (radiator, for example) that could be dismantled were sold for scrap.
All that time, the cannon was fully operational and just missing the firing pin, so the recovery team had to weld the breach shut and cut a hole in the barrel to demilitarize it.
To get it working again, the wiring, pneumatic starter, air and fuel filters and the fuel pump had to be replaced and/or repaired (the guys had the operational manual).
With the help of a crowbar (soviet engineering!) and the repaired pneumatic starter, they rotated the crankshaft a couple of times, then used to external batteries from a KAMAZ to start it up. Surprisingly, the engine and transmission were both in working order!
Afterwards they spent half a year fighting bureaucrats and government inspections to prove that this is in fact, legal. In the end, they won and got to keep it.