This is the "Rebel cannon sled", a piece of production design that attempted to maximize value by building it on a real vehicle. In this case, an AKTIV snow trac sled.
https://twitter.com/LiveActionSW/status/1432755142332272642
The blueprints for it call out the snow trac base as well as its use of an 'existing cannon unit.' 
It does show up in the finished film, if you know where to look for it.
As the hangar bay screen shot shows, it made it to both location shooting in Norway and on stage photography at Elstree. It was small enough to transport, I suppose.
You may occasionally find photos of Luke in his snowspeeder pilot manning the guns online labeled as a 'deleted scene'. It's not. It's more like a photo opportunity, as this prop / vehicle was on set that day.
* pilot outfit, that is.
Never been able to confirm what model of Aktiv vehicle is under there, but this is a strong contender:
Between the Shining and Empire Strikes Back, 79-80 was a busy year for Aktiv production vehicles.
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