The Marks Rack and the M18 front rack are two different models. The M18 only has a single set of struts, and is usually expected to be cheaper than the Mark's Rack. The Mark's Rack is also often listed as the "M1"
They are both great racks and fit on the same bikes, but if you want to double strut it, the Marks Rack is the way to go.
Bill Lindsay
El Cerrito, CA