These definitely are worth keeping! There are people out there who don't collect anything else

Interestingly enough, if your letter hadn't had this stamp on it, it wasn't delivered - on the other hand, if your letter was to be delivered to any territories belonging to the Soviet Union (like East Germany), the Soviet authorities would sort them out and refuse delivery.
From January 1st 1950 until March 31st 1956 you HAD to put these on every letter, postcard, etc. in whole West Germany. There are a lot of different watermarks, perforations, as well es plate errors and variants.
I don't know if you can see the pictures, if you are not registered, but here:
http://www.philaforum.com/forum/thr...hreadid=5882is a thread dedicated to the Notopfer stamps in the German forum I'm active in.