Go to the Historical Breechloading Smallarms Association (hbsa-uk.org/knowledge-and-research/) and check the WW1 Commemorative Film for :
Chandler Trench Periscope adaption of Long Lee-Enfield Rifle - demonstration.
Hope this is what you want !
“There are invisible rulers who control the destinies of millions. It is not generally realized to what extent the words and actions of our most influential public men are dictated by shrewd persons operating behind the scenes.”
Springfield Armory National Historic Site has an articulated stocked Springfield Rifle. This illustration was used in my second book (Sharpshooters: Marksmen Through the Ages)
BTW, during the American Civil War, Union soldiers at Vicksburg used mirrors to aim their rifles at the Confederaates and this allowed them to shoot without exposing themselves to retaliatory fire. By the time they reached Georgia, they drilled holes in their stocks and stuck the wire handle of a hand mirror for the same purpose. This predates the periscope rifles of WW I.