Unusual construction, no seaming.
The socks are started at the top with the rib. Stitches are set aside for the central panel on top of the foot. The back of the leg, the heel and sole of the foot are knitted. Then the stitches for the central panel are picked up and the panel is knitted, joining to the back of the sock as the knitting progresses. Then everything is joined together to knit the toe.
Written pattern with a chart for the stitch for the central panel.