All observations above as to why custom grips grips must be finished to the gripframe are correct. Polishing & finishing probably is the most labor intensive part of building a decent revolver. Even with two grip frames from the same mold, slight differences after polishing will be evident. The grip spike arrangement brought to popularity by Warren Center on his Contender, and continued by Dan Wesson and Bill Ruger, eliminates costly manufacturing steps (while expanding grip possibilities).
Either a custom grip is custom, or it is a slap-on.
David Bradshaw