In reply to:

sheth, noun.
1. one of the ribs of the framework for
the bottom or sides of a wagon.
2. the part of a plow to which the
moldboard and share are attached.

I have to admit I have trouble visualizing the structure of either item.

wwh, I've got Britannica up. There's a photograph of a moldboard, which isn't a board at all, but a curved, short blade that appears to turn the cut earth up into the banked row. I suppose it's ok to paste this from Britannica:

"In its simplest form the moldboard plow consists of the share, the broad blade that cuts through the soil; the moldboard, for turning the furrow slice; and the landside, a plate on the opposite side from the moldboard that absorbs the side thrust of the turning action.

So there are three actions going on in such plowing: the share cuts the wide band through the soil, the moldboard blade, by virtue of its curved plane, pulls the soil up in the long, banked row, and this "landside" keeps the banked-up long row nice and tidy by keeping it from spilling over from the moldboard.

Although there is no mention of the sheth in the article here, I would guess that the sheth is simply the arm to which the share and moldboard blades are attached.

The 'digging stick' is the eloquent name of the antecedent to the plow, according to the article.