Envelope Seals for German Libraries

Several years ago I acquired a small collection of envelope seals for German libraries.  Some of the seals are shown above along with an envelope which includes one of the seals. Most of the seals are for German university libraries (universitats bibliotheks), and most of the seals depict a heraldic eagle as the dominate feature. There's an interesting typed message on the envelope. It reads: "Hello, Sam! You may already have these. Eunice." I assume Eunice was referring to the stamps on the envelope and that Sam was a stamp collector which is probably why the envelope has been preserved. "Reichstag" on the seal on the envelope which was mailed in 1925 refers to the parliamentary building in Berlin so the seal was for the parliamentary library (pre Nazi era). I was successful in finding a website with some information on the use of German labels on envelopes. These seals are an example of one the many kinds of librariana that someone interested in libraries can collect.