Looking to run a 135mm quick-release or solid axle hub on your Surly with 12mm Gnot-Boost thru axle dropouts? These little dandies are the solution to your problem.
If you have a 2017 or newer Karate Monkey, Krampus, Troll, Ogre or ECR your bike has open 12mm rear dropouts designed for 142 or 148mm thru-axle hubs. To use quick release or solid axle hubs you'll need to install one of the appropriate washer style on the end of your axle before installing it in your bike. Voila! Magic just happened.
Looking to run a set of quick-release road disc hubs on your fancy new Midnight Special? These work for that also! Use the quick-release version with your 135mm QR hub as described above. Whoa, hold on there, Trigger! You're gonna need a set of the solid axle style for your front hub! Put them on the OUTSIDE of the dropout, make sure your skewer nut has sufficient threads engaged and tighten down. Got that? Use solid axle adaptor washers on the OUTSIDE of your dropouts for the Midnight Special's 12mm thru-axle fork. Easy.
The only difference between the solid axle and QR versions is that the QR variety has a capped end with an opening for the skewer only to pass through. Your QR springs will push against the cap on the washer, making wheel installation and removal easier. The solid axle variety will work on QR axles also, the washers just won't stay in place as well. Make sense? Good. Buy a pair, put them on, go ride your bike.