By "stiffeners" it means the J stiffeners formed on the edges of the skin.
This is one of those things hard to describe in words....
but if you position the bulkhead so that it is too far fwd on the skin, and then curl the skin slightly to get the "stiffeners" engaged in the notches of the bulkhead, the curl in the skin will make the 1486D clear the bottom of the bulkhead.
Then start sliding the bulkhead to its proper position and everything will drop into place.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")