Sorry, I am unable to tell you the name of the distributor that my FFL spoke with in regard to the 9x19 military P99 Walthers but, I had him specifically ask if the guns they had did in fact have the German proof marks and the distributor said that they did not.
I believe your FFL's distributor is mistaken. Or he doesn't know what to look for. Unless someone can produce evidence otherwise (pictures please), there are no 9mm S&W produced Walther P99 slides. Only .40S&W. They are easy to identify. Lasered P99 logo (rather than engraved) and no proof mark on right rear of slide.
James