This cast on is called by various names, Latvian (Joyce Williams, Latvian Dreams), double-start (Nancy Bush, Folk Knitting in Estonia), and double-crossed (Terese de Dillmont, The Complete Encyclopedia of Needlework). You will notice in the photo that it bundles the stitches two-by-two, however you knit the individual stitches. The pattern calls for 55 sts. I cast on 56 sts (28 bundles of 2 sts) then, on the first row, I worked a purl 2 together to reduce it to 55 sts. This is basically a long-tailed cast on technique. Two strands of yarn lay over the thumb to create the decorative appearance.
Hold the yarn in a two-tail cast on position — one yarn over your finger and two yarns over your thumb. Work the first stitch the same way you would in two-tail cast on with yarn over your thumb as shown. Dip needle under outside thumb yarn and pull loop up to finger strand. Grab finger strand with needle and pull it through thumb loop. Pull on thumb yarn to tighten loop gently at bottom of stitch. You are half-done. The second stitch will create a bar at the bottom of the two stitches.
Hold the tumb yarn as shown. Insert needle under the portion of thumb yarn near to your palm and pull it up to the finger yarn. Pull the finger yarn through the loop of the thumb yarn. Pull gently to tighten the thumb-yarn loop. The two stitches are bundled closely by the thumb yarn.
Always work a row of stockinette stitch (knit on the right side or purl on the wrong side) in each stitch on the first round or row. This makes the decorative effect stand out more.
Here is Estonian Cast On for Sock Knitting – KDTV 212 w/ Nancy Bush from YouTube: