How very handy this isn't lore and you can have several of them placed on your wrists (2), chests and legs. That's potentially 100 extra ac :-) I may have to start hunting here.
100 unadjusted ac, that is. Which slot you put it in matters. AC in chest gives about 1.5 times the amount in your total ac. Semantics I know, but for those that dont get all 4 as needed to finish these, you should put your first in chest, then legs, then wrists.