Al:
I don't know if the plinth is original or not. I was not considering the question of authenticity when I photographed it, but it looks like it's been there a long time. I've posted another two pics. The first pair shows another view of the left plinth, and gives us a look at the right one. They are pretty similar to one another. You can see a gefuig on the upper end of the right flute on the right side of the right plinth. Can't tell if it's a carving tool slip-up or from some later workman's efforts. The finials themselves are pretty distinctly John Townsend. The wood from which they're made is incredibly dense, dark purple. Almost no pores, and it's not filled. It must have been hard to carve. No sign of tool marks whatsoever. The carving is extremely precise, and has suffered little damage over the years. The uneven fluting of the plinths and the rough way they're stopped makes you wonder though. In one photo you can see the saw marks on top of the plinth as well as the x mark used for drilling the hole. It's also interesting that the flutes go all the way down. Designwise, is it better to stop them? What do they look like on the JT highboy that you measured?
JD