The SM is the Slider Futurity 1 - 1/4 inch - a sliding plate that is 1 1/4 inch wide at the widest part and has tapered heels.

The sizing refers to the amount of steel to make the shoe.
10 inch = 00
11 inch = 0
12 inch = 1
13 inch = 2

It is a steel unclipped shoe and packaged in pairs.

Available in sizes 10-13

Can be used with the Mustad Slider 4.5 Nail