The shims set the gear mesh depth between the face gear on the pump and the gear on the engine cam. This is critical for wear of the gear teeth themselves. This distance never changes - and is unique to the block once the surface of the block where the pump bolts up is machined flat in manufacturing.
The shims are roughly rectangular, with a large hole in the middle to clear the pump housing, and four holes in the corners for the studs that mount the pump to the block.
As for your genset having no shims, look again - it can be really hard to see the shims sometimes; my 003A had the shims stuck to the block when I removed the original pump and I didn't even see them until I looked closely. When the engine was painted, it blended the shims into the block really well.
