Hello Agus,
The 'circle' on the engine cover is for the attachment of a BTH generator for electric lights. Electric lighting was an optional extra on the early EW models, so some bikes had a plain cover, whilst others, like yours, had the modified cover for a generator. These generators are often known as 'Pancake dynamos' because they are short compared with their diameter. From the photo, it looks as if the hole for the drive shaft has been plugged to prevent dirt getting in - inside there should be a special gear to drive the generator.
From the photos of the frame and engine numbers, I see the rear frame stay is not the original and has probably been made up by a previous owner - the end of the tube would not normally be open.
If you are lucky enough to buy this bike, post some more photos and then we will be able to give you more accurate advice. When the EW was first produced, quite a few problems occurred which resulted in the factory making modifications to the engine - including a modified oiling system, with more rerouted oil lines, so some illustrations may not match exactly to your bike.
Regards,
Eddie.
