Water pump dates

[l79m20]

Am just finished changing out my water pump for a 250 cu and am wondering if the pump taken off is the original one to the truck, I am second owner. On the front of the pump is cast into pump GM7 and the casting numner on the pump just above it is 3788478. What I assume is a build date for the casting is a circle with an m in the center then G309 another circle with a 1 in the center. Can any decode this for me and do i have the original pump, will save it to be rebuilt at some other time. Any thoughts? This is a great site!!!!.............................Kirby