So more info might be helpful, such as the assembly date of the engine (pic 1 below). It's on a "tab" on the engine block, the yellow arrow, near the distributor (The red arrow points to the ID number for the HP head.). It's in the format you've called out. If the engine assembly date is before the manifold cast date, then the manifold was cast after the engine was assembled and should not be used. There is also an engine cast date near the starter motor.
Jim