This chair is extraordinary in more ways than one. It's built on Clissett's spindleback frame which differs from the usual ladderback in having no finials, vase-shaped front feet, and entirely different arms and under-arm turnings. In addition, it's stamped "PC" on the top of the front legs - the normal ladderbacks are are almost never stamped, and can't be stamped in this position because of their finials.
Furthermore, the slats on this ladderback are very different. Although graduated in terms of both height and width of the central part, this graduation is nowhere near as marked as normal, and the upper slats have less height than usual. And, whereas each slat would normally be shaped towards the top to create a blade-like edge, these are entirely unworked leaving a square edge along their entire length.
It's tempting to speculate that this chair is the forerunner of the famous ladderback, tweaked by James MacLaren, bought by many Arts & Crafts practitioners and followers, and still gracing the Meeting Hall of the Art Workers' Guild. There are similarities to a chair made by Philip's uncle which adds some credence to the speculation. But there is also some evidence that Philip made chairs that mix the styles seen in his spindlebacks and ladderbacks. So we're left, yet again, uncertain of exactly what we're looking at!
Many thanks to Paul for letting me know about this chair, and giving me an opportunity to have a good look at it (and for letting me use the photographs).