The early 30 round magazines had a full seam the later a partial seam. If you go to the link Charlie added here in post #3. Then look at the pictures in post #1 by thorin6 you will see the differences.
My bad, I wasn't being specific. Jim, you are saying that the hardback mags had the full seam and the split back had the partial seam, right? I failed to note that I was asking only about the splitback mags.