I used to have a late 30s BSA Sports and this bike is quite similar. I did a lot of detective work on mine so I might as well pass on a bit of that knowledge here. I'm glad you found a Hercules marking because there were a lot of these factory-built Roadster and Sports bikes made by Raleigh/Triumph/BSA/New Hudson/Gazelle/Hercules etc and they can be difficult to identify once they lose their badges and decals.
They built bikes like this pretty much unchanged from the 1930s up to the 1970s. To my eyes the frame, fork, bars, stem, brakes and mudguards all belong together. Saddle is obviously later and doesn't belong with it. The headset is a clip type headset. These were generally obsolete by the late 30s but they do still pop up on later bikes. The handlebars are Lauterwasser bars which were a popular shape in the late 30s and 40s and on your bike they're upside down. The brake levers and calipers look similar age, so that's why I think this all belongs together. Everything looks like it has black paint slapped over it. In general, if the bars and stem are plain painted steel it dates them 1939-1945 because there were wartime chromium shortages. If they are chrome plated under that paint it dates them pre- or post-war. The metal mudguards fit well, so I think they belong with the frame. Frame has quite slack angles which would be consistent with a late 30s frame. The frame has fittings (like the pulley under the seat cluster) to run a Sturmey Archer hub at the back, but the rear stay also has a mounting point for an early 2-bolt direct-mount derailleur like a TriVelox B or Cyclo twin cable.
So. If I was really pushed to say what I was looking at here, I'd say this is a late 30s or early 40s bike which was 'updated' at some point with the Sturmey Archer rear and Dynohub front for the light, and the bars were flipped for a more upright riding position. Since the hubs are dated 1948 and 1951, this was probably probably done in the late 40s or early 50s.
