The F-721B L&R get modified from the way Van's send them. Basically, you cut a 3/16" wide slot in one end that's defined by some pilot holes. If you're building a tip-up, you follow up by cutting a section off the end, but if you're building a slider, the slot is the only modification. Connected the dots of the pilot holes freehand on my band saw, then opened them up to 3/16" with a jewelers file. Accurate width was defined as being able to slide a #40 drill bit back and forth in the slot without it binding.