This is an unfinished model of the Nona-S self-propelled mortar. Originally, self-propelled mortars were intended as a separate class in the game, offering short-range indirect rapid fire with low damage per shot values.
The first ideas about this class came from Obsidian Entertainment back in 2013-2014 and included multiple vehicles of this class, such as the Czechoslovak PRAM-S or the abovementioned Nona-S. However, the concept was not publically tested until years later and, following poor feedback from the community, it was shelved for the foreseeable future.