petethekiller,I am looking at an old back street heroes magazine at an 'Uncle Bunt' frame advert,his  rigid frame in
the ad shows a 'center post',also the 'axle plates' are nowhere near what you have on your frame,it also has a
battery box mount behind the center post,that battery mount is made of  a  ''rectangular section' not box,or tube.
I believe this is one of the "trademarks" of a true 'Uncle Bunt' frame,I have a friend with a true 'Uncle Bunt'
framed triumph this too has that 'rectangular section' battery mount.....hope this is of some help..sh..in the UK.