My apologies for any misunderstandings on what I noted these to be. I based my comment that they were "spacers" on the picture Paul posted which showed no threads in the bolt holes of the 3 plates (from what I could see and the holes appear to be larger than the screws nearby). It makes total sense that these would/should be threaded anchor plates as noted by many replies above, and that they need to have the serrated faces to grip the latch plate mounting panel when properly tightened to keep them from moving for door latch function and security.
Attaching a pic of an NOS GMC latch plate set I found on eBay last night below. The 2 plates show different serrations, 1 lengthwise and 1 crosswise.
Once again, my apologies for any misunderstandings.
