I'm learning here as I have the same M28-2 with serial # S2834XX that has the diamond target grips but not serial numbered to the gun. I had made the assumption they came with the gun but perhaps they were changed out later during that same time period.
Would that be correct or would have they possibly been sold with those grips but not numbered?

Mine is about 90-95% percent and paid $250 for it a few years ago which I felt a bargain then. It's a shooter and field carry gun as I have a nicer 28 for looks and range shooting.