I am definitely no expert on this, but it seems logical to me that different factories might have slightly different neck stamps. I doubt Mattel has a large inventory of old Pleasant Company dolls they are selling. It's possible that different factories produce dolls with subtle production differences - like how one doll might have really pink cheeks and lips or another might have almost no coloring; or why one might be stuffed very full while another is much slimmer, could be attributed to the dolls coming from different factories. That would be my guess.