Perhaps there is an error in the 1.0.3 UML definition of the OBJECT_REF class: in the 1.0.2 what was named as "namespace" property appears now in 1.0.3 as "id_namespace".
I didn't find any reference about this change, so I don't know exactly if it is intentional or a typo.
For reference see:
https://github.com/openEHR/specifications-RM/commits/master/docs/UML/classes/object_ref.adoc
http://www.openehr.org/releases/RM/Release-1.0.3/docs/support/support.html#_object_ref_class
earlier version of specs:
http://www.openehr.org/releases/1.0.1/architecture/rm/support_im.pdf
http://www.openehr.org/releases/1.0.2/architecture/rm/support_im.pdf
This seems to be a typo. I've fixed it, and created a CR for RM 1.0.3 typos.