The author of this page is in contact with Morgentaler of non jewish background in Canada and Serbia. He is in posession of a passport copy from Serbia of a Morgenthaler which shows that in 1931, the letter h was dropped when issuing the document to Morgentaler in handwriting. It can be assumed that this is the root of the name variant Morgentaler without th in Serbia.
The reason of this omission is probably the fact that Serbian is written in the cyrillic alphabet, which contains no h. (Explanation by Inge Morgenthaler, Germany)
Please contact the site author if you have additional, different or new information about this subject.