The difference is a function of the manufacturing date.
The model 50 had minor variations of the slender stock for most of its life. But by the early 70's, Diana's classic rifle lineup (models 15, 16, 22, 23, 25, 27, 35, 50) was getting pretty long in the tooth, so they did some updating to the top models. The models 27, 35, and 50 got the squared-up wood (quite the rage at the time), an auto trigger-blocking safety, and other tweaks. The new barrel cockers were the 27S (see my pic just above) and 35S. IIRC, for some reason they didn't change the 50's name, but the design cues are the same.
The 27S and 35S were pricey and not successful; the preceding "plain" 27 and 35 stayed in parallel production and ended up out-living them. The 50 morphed into the model 50 T01 in the late 70's, based on the model 45 barrel-cocker's action (earlier 50's shared powerplant and trigger with the 35). The 50 T01 was a very hot tap-loader, would do over 800 FPS.