This is made to be a bottom bracket lockring for an old style cup-and-cone bottom bracket...but it has other uses:
If you want to convert an old freewheel type hub to fixed gear on the cheap, using one of these 1.37 x 24 TPI lockrings will help secure the sprocket from unscrewing. It's not as secure as a proper track hub, where the lockring is left-threaded, but it's better than nothing.
These lockrings are also useful for converting a Uniglide Freehub for singlespeed use.
