U was the Unciform bone of the wrist.
"Punch, or the London Charivari, Volume 1, Complete" by Various
U, The unciform bone.
"A Treatise on Anatomy, Physiology, and Hygiene (Revised Edition)" by Calvin Cutter
The final bone of that series, the unciform, seems to represent two bones fused.
"The Cambridge Natural History, Vol X., Mammalia" by Frank Evers Beddard