As Peter has said, the 2 photos show British Hub brakes (they are the ones with spun sheet alloy brake plates). The earlier Douglas brakes have cast alloy brake plates. It was easy to swap early for late rear wheels, as they were the same width. That is not the case with front wheels - the British Hub wheel is wider than the Douglas wheel, so later bikes have a front fork that is wider between the wheel bosses.
Also, from Peter's photos, I have noticed that his front wheel has been fitted with the wrong rim - it is one from an early wheel (or a rear wheel). As the later brake plate covers the front wheel spoke flange, all spokes (leading and trailing) have to be laced from the outside inwards. In order that the leading and trailing spokes shouldn't bend around each other (and chafe), the piercing and dimpling on later rims was changed from the normal staggered pattern to 'one left and 3 right'.
Kevin, As far as I am aware, the change from Douglas brakes to British Hub units originally took place around frame number 1278/6 - of course, that doesn't mean subsequent changes haven't taken place!
Hope some of this helps,
Regards,
Eddie.