If you haven't worn the shoes out completely, you should be able to find inside each shoe a tag with a upc barcode as well as a number. That number is the identifying number for the model.

The model number (or SKU) of a shoe can be found on a label located on the inside of the shoe tongue. RN stands for registered identification number. It is a number assigned by the FTC to US businesses that manufacture, import, distribute or sell products covered by the textile, wool and fur Acts.