The ß in my Name originates from the letters s and z (and is called "sz" in german). For EDV means it normaly translates into "ss", since sz looks kind of weird (but would be correct). I don't realy like it because tehre is a diffrence between Theiss / Theis and my Family: Theiß
But that is the only letter i know about a thing like this. For the "th": A lot of english / american speakers pronunce my name with english "th" and read the ß as a b so it sounds like "thib". We say Theiß like Taiss (sorry, would have to look up the phonectis and how to write them) The th is spoken more like a single T or maybe a D, but never like "th".
Could be that someone changed his Name from "th" to T or D in the beginning to avoid this... it just sounds strange to pronunce it "th"... But that would not have taken a new letter to do so. ß is the only "combined" letter i know. But i am a scientist and not a student of phonectis or languages...
P.S.: Whoever finds spelling mistakes in my post may feel free to eat them, or decorate his christmas tree with them i don't about spelling mistakes!