If I remember correctly, only two were built, either that or only two saw combat.

It's very low rate of fire is it's achilles heel, it's like one round per minute. In combat mission you should be sure that all of your oponents tanks and at guns are dead before you roll it out of hiding.

The blast radius from that 300mm mortar is insane though, it can kill two tanks with one shot.

In real life the mortar was actually an antisubmarine weapon, unless I'm not remembering correctly.