Can anyone tell me what the GM number cast below the head number means?

My head cast: 3835499
GM 9

http://s1142.photobucket.com/user/52_Ton/media/100_2536.jpg.html?sort=3&o=2

I have found a head with GM 7 on it.

Mark

Last edited by 52_Ton; 09/13/2014 2:14 PM.
1952 GMC 9430 one ton pickup, a work in progress

1952 1-ton pictures on Photobucket