There were several German patents for recoil reducing devices built into buttstocks. Some examples:
DRP 8519 of 1879 by August Leue, Berlin
There was an 1898 design by Sempert and Krieghoff that I have not found yet.
DRP 167737 of 1904 by J.Strehlow, Breslau
DRP 168327 of 1905 by Burkard Behr, Hamburg, similar to the 1960s Winchester Hydrocoil.
DRP 187718 of 1906 by W.Deditius, Breslau
You may find the patents here:
https://depatisnet.dpma.de/DepatisNet/depatisnet?action=einsteiger
Just type in DE and the patent number under "Veröffentlichungsnummer", scroll down, click on "Recherche starten" and go to "Originaldokument pdf" Usually you find the drawings on page 3 of the patents.