It's that little rusty strip of metal amidst all the other non-corroded stuff.
If you lost it, no sweat...it's only a buck fiddy six from Van's:
Code:
SS4130-050X1/2X20 F-459/F-6119/F-693 $1.56
And yeah...imho, wait until you have your actual seats, cushions, rudder pedals installed permanently, cables rigged to the rudder, etc. Otherwise you'd just be guessing what the length should be.