Yes indeed with the annexation of Austria in 1938 there does not seem to be many different letterings.I like your triangle very much. Longer time I was seeking for a "Südost Salzburg" and finally got it for my collection. It increases the value of your shirt.
Due to the fact that Austria became part of the Reich only in 1938, we don't find any major differences in production or lettering today. It would be interesting to find out whether there is a particularly early form of an Austrian district sleeve triangle.