I also think this was a clever way to save fabric and fixate the side straps.
....
A ca. 1929-1933 navy blue DB blazer with lightly padded, round shoulders. The original buttons were damaged, so I replaced them with matching ones.
It fits me perfectly and has long sleeves.
The label points to Friedrichshafen am Bodensee (Lake Constance between Germany, Switzerland and Austria).
I imagine that it was worn with white flanell slacks aboard a ship or promenade.
What exactly do you mean? It is a strip of fabric sewn on the inside under which a elastic band, affixed to the side seams, is tunnelled.