This is a drive-side stop washer for your Surly Mr. Whirly Crank. It ensures you're properly spaced out down there.
This OD Crank Single Speed Spacer Kit gets your Surly OD Crank ready to run as a single. Indluces chainring bolts and spacers. Just add chainring (sold separately).
Surly offers you spiders for their Mr. Whirly Cranks in a variety of Bolt Circle Diameters (BCDs) so you can run nearly any chainring combination you want. Be sure to check chainring/frame compatibility for your intended setup.