That's not a typical cassette hub, but is a freewheel which uses a lockring to hold the sprockets on the freewheel body. There aren't that many of these around, which means that finding the right tool to engage the lock rings inner splines difficult. The lockring needs to be seriously tight so it can't come loose. You might also benefit from using some Locktite to keep the lockring in place, but if you go that route, make sure the ring and freewheel threads are clean (really clean) and dry.