Cassette lock rings?
 

I have a Shimano 9-speed HG cassette that needs to go on a wheel, but I need to get a lock ring for it. Can I use any Shimano HG-labeled lock ring or do I need to get a specific one. E.g., would a HG-50-9 be basically the same as a HG-70-9 lock ring? Thanks.

Yes, these are interchangeable. There are some lockrings that will not work with 11-tooth cogs but those are becoming more rare.

Thanks!