I just bought an old Mark Richards Twin Fin for cheap at a garage sale, but I can't figure out what type of fin system this takes. It looks kinda like future fins, but there's no screw hole. It's got two small bars on either end of the fin box that a fin would "lock" into place on, but that's it...it's got me stumped. Any help would be great. There's a picture attached to this, if I did it right...first time on the forum

Thanks!