The offset between the tappet and the valve stem is a design to allow the valves to be further outboard than the tappets. It keeps the timing chest narrow. In the case of the Douglas the offset does not impart a rotation to the valves. Usually that is facilitated by having a round tappet foot offset to the cam lobe. That causes the tappet to rotate slightly, and imparts some of that rotation to the valve. The Douglas tappet is prevented from rotating. If the valves do rotate, it is probably more due to accident than design; via the head of the lash adjuster flexing, causing an angular strike on the valve tip, that might induce some twist.
Valve rotation might have been known in the industry at the time (I am not sure without doing research) to reduce or even out wear, but I don't think Douglas knew about it! Or if they did, the layout of the valve train (inherited from the EW model) precluded it.
-Doug