Why are these spec and serial numbers blank on a part's plate of a WWII US military plane?
I suspect the reason for this has nothing do with aviation and better fits at our history or statistics stackexchange counterparts. 

Information about the production capacity of the enemy is of major strategic importance. Using the serial numbers of a relative few aircraft one can make a quite accurate estimate of the total number of aircraft produced. This was found out during world war II and is known as the German tank problem. 

What I suspect is that after this methodology was developed, producers were asked to withheld the serial numbers from their products and/or replace them by a cryptographic equivalent.