The drain bolt is at the bottom of the down tube, below the outlet pipe. Lay under bike and it's in the middle of the tube, approx 10 mms bolt. I fill both of mine until you can just see oil in the top tube, then run the engine for a few minutes to get all the oil up and check again. I then fill until it's just under the return pipe, but don't worry if you go too high as it just comes out the breather. Cheers
The above is for a genuine Foster Otter, not sure on the copies.