"Superceder" was the brand name that my brother and I used selling barrel tapered Mississippi yellow poplar shafts more than a decade ago. They are spined by the old standard, a 2# weight and 26" centers. What works in spine for other wood shafts will work for these. The mass weight of them is intermediate between softwoods (cedar, pine, fir, etc.) and the heavier hardwoods (ash, maple, hickory).
I have about 100 dozen left, mostly 35-50# and 65-80#, plus some heavy weights. My ad for them was pulled because I had too many for Trad Gang's classified policy. I've been selling them instead on "that" auction site.