On OK, and OL they are reinforced by heavy steel plates with notches on the top and bottom of the side sills. As the main guide slot is a vertical slot cut into the wood side sills, these plates provide a tougher wearing guide to keep the wood slot from wearing out. Follow the link below and look at the second photo. The plates show up in the top and bottom of the side sill. Check out the rest of the pictures at this site for more views of the flanger restorations at Chama last year.