Bikemonger says... Interchangeable dropouts for some older Surly frames. Notably Karate Monkey Ops, Krampus Ops, Instigator 2.0, and Ice Cream Truck.

Surly says.... The 12mm Slotted MDS Dropout is the chip system employed by some of our legacy trail bikes. You like chips, don’t you? Who doesn’t? The 12mm Slotted Dropout Chip uses a standard derailleur mount for maximum compatibility and is made of cast and machined 6061-T6 aluminum, with a fender eyelet. When you get an MDS-equipped frame you’re getting the option to set it up however you want. You can run a 142mm x 12mm thru-axle rear end with a direct-mount Shimano hanger, standard 135mm QR or The Mighty Singlespeed.