In the Great War, the Germans developed the stick grenade, where a wood handle screwed into an explosive head. To make shrapnel, a sleeve called a splittering could be put over the head.

I think the "paddle" was an antipersonnel mine. The casting resembles a splittering. The first crew would go along 50 yards in front of the trench using a pipe and hammer to make holes in the ground the right size for the paddle handles. They'd insert a handle and tamp earth around it.

The second crew would put grenade heads into the splitterings and stretch trip lines.