Hi,
There are a few books on German philately, printed around 1900 or before, that use the word "Postwerthzeichen" (meaning postage stamps in English) instead of the far more common spelling "Postwertzeichen". I have included two examples below.

It is clearly not a spelling mistake so I would like to know if this is simply an uncommon, but valid alternative spelling, or maybe it is a grammatical twist of some sort? It is possible that the word, being only a few decades old, did not have a settled spelling yet, but there are miles of other publications from around that period all with the "Postwertzeichen" spelling, and that is the only spelling I can find in the dictionaries, however old.
Lastly, I need not point out (but I will anyway) that I know very little German, and so my guesses may be completely off track. Whatever the cause of the additional "h" I would be most grateful for any insights.