Why are these spec and serial numbers blank on a part's plate of a WWII US military plane? These pictures are of a plate from WWII US military plane that crashed in the high mountains of central Japan. 

People living there say that after the war officials recovered the crash sites and some parts were put on display in a museum nearby.  This plate is attached to a small metal box which is part of the museum collection.

It says "gear box type JH-1045" manufactured by "Jack &amp; Heintz, Cleveland Ohio".

But SPEC. and SER. are blank.  I assume these are where the spec number and serial number should go?

It looks like somebody carefully cut those parts off the plate.  People there say that was not done after the recovery, so the plane was flying with this plate as it is.  Nobody remembers the type of aircraft nor where the rest of the wreckage went.  I do not know whether the numbers were scraped off before flight or were just originally not there. 

Why are these spec and serial numbers blank on the parts plate of a WWII US military plane?

Regards,
Naoya
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.