Munster, I was at one time going to build a P1 back in '08 and also noticed no mention of dihedral. I got in contact with Don Lowe through his son Jon and received this message:

"for the life of me I can't recall the dihedral--it had some probably about 1in. under each tip. As you know with sweep as you pull angle of attack dihedraleffect takes place. The more alpha the larger the effect. Thats why over theyears I reduced the sweep. Any wing with some sweep at the quarter chord willexhibit some coupling."

I did scratch build a wooden P5 and used the 1" under each tip number as per its plan and it exhibited very little roll coupling, but it's a drastically different sweep and fin angle layout from the P1. I'm glad you're taking an interest in SPA and Vintage Pattern in general and I invite you to come down for a contest sometime with Mark. Which reminds me, Mark, get your butt down again too!
Eric