SirMike1983
Riding a '38 Autocycle Deluxe
My experience is they usually are found on early and mid 1960s era bikes. The purpose is to prevent the drive side front hub cone from tightening itself as the wheel turns forward. Earlier Schwinn hubs often have a thin lock nut that holds the drive side front cone in tension so that it does not turn with the wheel. The British method of solving the problem was to machine a ridge into the axle so that the drive side front cone could not turn any further once bottomed out on the axle ridge. All three are meant to prevent that cone from self-tightening with the forward turning wheel, and thus self-destructing the hub.