I would suggest that the original spelling of this name was Thurnwald. In earlier days in German "th" was used instead of "t" with exactly the same pronounciation as "T" - e.g. "das Thal" (the valley) nowadays "das Tal" (the valley). In addition the "d" at the end of a word has a much sharper sound than the English "d" and at the end of a word sounds more like a "t". The variation in the spellings probably came because he was trying to simplify things in an English speaking country where these points were not known. Although he was born in Czechslovakia, which was created in 1919 from parts of the Austro-Hungarian Empire, there were a lot of German speaking people there at that time and the name is obviously German not Czech. The Czech Republic only came into being in the 1990s when Slovakia split from Czechoslovakia and two republics, Czech and Slovakian were created. Hope this helps a bit.