I'm gonna guess fiberglass 
Give the man a cigar!
It's laminated FPR, fiberglass reinforced plastic.  I was trying to get piano key overlays, but didn't score. 
The downside, is that I was trying to take photos of the bow with full draw and now the tiller looks drastically different.  Both limbs hinge right at the fades.  Yeah, I know..I know!  This is what happens when you use 
EFFING-GLASS ON A PRIMITIVE BOW.I talked with iowabow about it this afternoon and he suggested I heat treat both fade outs, adding 2 inches of set-back to the handle, and try taking a very few light strokes off mid-limb to bring it back to an ellipse.