Finally printed both pieces for the wrist bearing. It is constituted by 2 similar pieces placed symmetrically in order to compensate unbalanced loads. Each piece has 2 rails of steel ball bearings plus a centering  feature. Once assembled movement is smooth and wrist pivot seems to be placed in the right point.

Now there will be the attachment with the hand piece to double check that everything is in the right place and proportions. Meanwhile here is a quick preview of the bearing design and functionality.