Are you sure you are looking at Toe plates, or at the Heel plate? I see lots of heel plates made of bone or antler, on both original and replica guns. Toe plates, if they exist, are usually made from sheet brass, or sheet steel. Not all originals had toe plates, particularly if they had no buttplate, either.
Because of the way guns are often stood upright against walls, or corners, it makes more sense to put something stronger than wood at the heel, as with the stock pitch, the toe of the stock is often lifted up off the floor when a gun is leaned against a corner, doortrim, or wall. If the heel plate is squared at the back end, it lends stability( just a little) to the gun as the muzzle of the octagon barrel is also leaned against the flat edge of the top flat.
My Poor Boy Tennessee style rifle has NO buttplate, but has both a heel plate, made of antler, and a toeplate of steel.
In the same vein, the first custom Southern Rifle I saw had these features, as well as a muzzlecap made from antler. Very pretty piece. :hmm: