Early 1930 three-stay frame

-SG- Keith in New Zealand has what is probably a very early thirties three stay frame although I cannot find the number in the records.  He has asked if I know the purpose of the extra lug on the right hand chain stay.  It appears to be an original fitment and frankly I have no firm idea. Further, the anchor point for the rear brake is not central on the relevant vertical frame member but very much lower down. Has anyone any suggestions as regards the extra lug and/or comments re the brake anchor position?