Reply by Ben

Thanks huff.

No, it’s OK. too late at this point to swap slides. I’ll just do the 21” box and it will be fine.

I guess I’ll slide my first box up against the prongs and use that to layout the holes. Is just a round hole for that thing, or a special slot?

On Blum’s site they have an arrow pointing to the prong, and another picture showing the jig, but that’s about it.