On page 53 of the Skerry construction manual it says, "Frame #1 goes BEHIND the mark on the bottom plank, #2 & #3 go in FRONT of marks on the bottom."  This PROBABLY means #1 goes AFT or ASTERN of the mark and #2 and #3 go FORWARD of the marks, or toward the bow. 

But it could be interpreted to mean that Frame #1 goes behind the mark as one views it from inside the boat, or forward of the line, for example.  I am about 99% sure that Frame #1 goes AFT of the line and #2 & # 3 go FORWARD of the lines, but I want to be 100% sure before drilling any holes.