Two other points indicate also that the code is real:
1. The WL-xxxx code is just a document of transfer of operators (D-xxxx = civil aviation, WL-xxxx is military aviation).
2. Just the prefix (for the operator) changed. The code itself remained. There are some He70 in the range D-/ WL-Oxxx. So, even there might be no proof for the existence of a particular code, the code itself is at least plausible. The application of the "new" code itself may just be a differnt story.
Another example where the "D" has been over painted with "WL", the "WL" looking a bit "squeezed" in front of the Balkenkreuz. Perhaps WL-OZUS, previously D-OZUS, which can be found in the WL-???? register at LRG.