340 #'s

I recently purchased a '70 340 Dart Swinger. The original owner was in the Military and special ordered the car. Six months later he sold it to the second owner who kept it until I bought it. According to the second owner the car is all original. But when I pulled the motor the VIN wasn't stamped on the block. There is a spot for it on the oil pan rail, but it does not have a number stamped on it. It is nice and smooth.

Was it common to miss stamping the number on the block? Has anyone heard of this happening?

VIN- LM23HOR156897
Cast Code- 2780930-340-6
Date - 7-10-69