Bramble, It might be an original, it might not. Please post a photo or two and I will venture an opinion. A lot happens between 1918 and 2004 so the story you are getting may, or may not, have any validity. I tend to agree, that if the story holds true, it is probably an original holster. One thing collectors are always wary of is the story....Always look at the item, never the story.
Early WW1 holsters started out brown in color and it was later ordered that some German equiptment be dyed black to satisfy the High Command uniform regulations. Yours might have been captured before it was subjected to this requirement.
It would be helpful to see a closeup of the monogram makers mark, the 18, quite possibly, is the date of manufacture.
Welcome to the Forum and I will look forward to your photo's. Jerry Burney