the pictured receiver is a hesse. you can't see any hardened areas because the receiver has been black oxided.
a milled parts kit does not include a trunnion. one must be purchased if you are going to assemble a milled parts kit on a stamped receiver.
early, threaded ak-47 milled parts kits must have the threads turned off te bbl. and fitted to the trunnion hole.
later press fit milled bbls. may or may not need fitted to the akm trunnion,