What are the letters and numbers etched into the portion of the vinyl(lps) between the last song and the label

Question

I just started collecting lp's, and I have noticed that the ones i have looked at have things like "SP-3211 M" etched or carved into them on the area of the vinyl between the last track and the label. From the appearance of the characters, it is evident they were done by hand. Im 28, and didn't really grow up with records, so I'm not familiar with them.

Answer 1

http://eil.com/explore/guide/vinyl_making.asp