Unusual (to me) German 20mm MG151/20 headstamp

In the late 1970s I had an opportunity to obtain quite a few examples of MG151/20 cartridges in three different loads. These were in the original cases and the owner had a case of each. All were in new condition. Today I ran across the half dozen or so I have left and glancing at them noticed one round where the manufacturer code (wa) was missing from the headstamp. This round is otherwise identical in all respects to the other rounds in the case. I have also illustrated the normal headstamp on the rest of this type load that I obtained.

Any ideas on the missing information? There is absoultly no evidence that I can see, even under magnification, that anything was removed from the bunter.