Below is the info supplied to me:
"This thing is named a Sperberroehrchen. Now, a roehrchen is a small tube, while a Sperber is a rueopean sparrowhawk. Either this attachment was named for the keen eyesight of this bird (there was also an Adlerauge = eagle eye attachment for a front sight tunnel), or the inventor's name was "Sperber"? I don't know. In German target competition scopes were not allowed, but elderly or shooters with defective eyesight could use "optical aiming aids", i.e. lenses attached to their sights. So someone designed such devices which also provided some magnification, but stayed barely within rules. Postwar, even some tiny "non-scopes" like the Hensoldt Diapi (see GGCA bulletin #35, page 6) were made for this purpose."