400 id

[0X01B8]

I've got a 400 motor with some conflicting numbers on it, maybe you can help.  First off, the pad by the dist. reads H400  7  7  E.  So, it's a 72 assembled on 7/7.  Happens to be my birthday so that's cool.  Casting date is something like 4-1?-7?.  The casting # is 3698630-400-5, which I thought was supposed to be for 73-up motors.  The pad where they stamp the vin has two faint numbers on it and nothing else.  Was this just built really late in the 72 model year?  Generally, when did they switchover to building for the next model year?