On my Haenel Modell 1900, see photo, the panels are exactly parallel, but IMHO this was open to the taste of the individual stockmaker, as is the exact shape of the side panels. Yes, they served a practical purpose also: About 1900 German sporter stocks were very slim. So the stock sidewalls around the magazine cutout were thin also and prone to flexing under recoil, which led to stock splits. Side panels stiffened this area. On the other hand, they were a leftover stylistically from muzzleloader days, as is the wedge fastening of foreend to barrel. On many muzzleloaders a side panel provided space to inlet a flat lockplate. When such muzzleloaders were converted to early bolt-action needlefires, the now unused lock recess was often filled in with wood, leaving a side panel. This side-panel feature remained in use by the Suhl gunmakers well into the 20th century.