As you say, the ground could well have been harder (or more abrasive, or the one I saw could have been used more extensively). But the toe of the buttplate in the picture doesn't appear to show the expected wear patterns typical of tools used in the ground, or as a fleshing tool. The tip of the comb end still has toolmarks, so that probably wasn't a hoe or scraper; wear-patterns would show up on the toolmarks first.
Going a bit further, I don't think it has been shortened at the toe--or if it was, it was fit to a gun after that. If you look at the third picture down, there appears to be a chamfer of some sort around the inside edges of the "butt" part of the buttplate, where the edges would be fit to the wood of the stock. Maybe it was used on a gun built from salvaged parts at some point, or cut down for a kids' gun.