It's probably a Uniglide cassette, which as HillRider says is held on by the threaded small cog, not a lock ring. You may just be able to see the threads on the freehub body if you look carefully at the very end of it. Two chainwhips are generally used to take a Uniglide cassette off a hub - one to unscrew the small cog while the other immobilizes the freehub body and prevents it from turning backwards.