ArsenalM resin version of the Karl Mortar.
My friend Randy at Fidelis Models sells them. https://www.fidelismodels.com/product/ArsenalM_112100021.html
They come in the short barrel 600mm version.
And the long barrel 540mm version. The longer barrel and narrower diameter gave longer range.
This is the Panzer IV munitions vehicle.
It can be built with the ammo space open or closed. They are excellent kits. I have two more waiting to build, in HO scale.