[bpick]

I recall reading that the same kind of thing used to happen with standing field armies, (think Napoleon sized blocks) where you could almost instantly assess an opponents forces based on simply recognizing divisions, regiments, etc.

My guess is Napoleon-era field marshals did a cost-benefit analysis and decided that managing their forces simplistically (i.e. no irregular sized units) outweighed whatever inconvenience enemy intelligence posed.

Relating this to the OP my guess is the Germans made a mistake and overlooked the serial numbers, rather than simply not caring.