It may have been a service/replacement block that came without a serial and the mechanic that did the swap stamped the old motor's serial as best as he could...?
The actual block casting number is below the distributor or dip stick.
The G80 is probably the casting date: G=7th month=July, 8=8th day of month, 0=50 => July 8, 1950. CONV1 is conveyor #1, or line #1 in the casting plant.
