I used a file to make it fit in the lock and it works very well on my KM20 toolbox.
The lock in my toolbox was stuck, I have completely dismantled, cleaned and greased it. I have then put it together, I made several photo's during the dismantling but can not find them at the moment. The lock itself is a very simple construction and easy to repair.
From 1940 onwards, the Tools and Equipment lists for the WD16H ceased referring to a WD replacement key source and simply stated 'To be purchased locally'.
I think therefore that it would be difficult to have a 'wrong' key.