Shimano Hyperglide-C (C stands for "Compact") cassette lockring
If you don't understand about the Hyperglide compact cassettes, read this before purchasing:
You can use these on existing compact cassettes or if you are converting a cassette with a larger than 11T top gear to 11T.
Oddly, Shimano doesn't put the part number on this part - but doing a bit of looking around the web,
it appears this is part number 10Z0300 and is for the Deore XT CS-M737 8-speed cassette with 11T-28T cogs.
It comes with a very thin spacer shim as well.
These were purchased in bulk so there is no retail packaging.