thank you so much!I think it was a serialization of the different molds used to cast the head. In your example, mold "C" and "E(?)" were used. On blocks they used numbers like 2468130-6 for a 59-71 383. The "6" is the mold serial number. Realize they were casting hundreds of these every day for years. I think I remember Chrysler making over 3 million 383 blocks!