They've got nothing to do with warbows! You'd never find one on a warbow. Tiny, simple nocks just from the end of the cow horn is what you want - no shaping, no carving, no designs.
The nock you posted would be a Victorian target bow where elegance and style were far more important. A warbow was a tool and was simple and deadly, in the same way as medieval swords wouldn't have been decorated or finished with gold inlays etc.
All that being said, they are string keepers. You just tie a loop of string or elastic to the nock through the hole, and the other end is tied to the bowstring loop to stop it slipping down. Warbow strings would be stored off the bow, wound up and hung.
