I have an issue with a freehub on my 1980s vintage racer - the cogs came loose when pedalling and now that I look closer, I think a part may be missing because the rings can now just slide right off. Thought I'd slap on a new cassette since the old one was so worn anyway, but can't find one that fits. It's a shimano sis system, but there's no lockring that I can see, just a some kind of plate with two holes that a pin spanner might fit. I clearly have no clue here. What to do?