The way I understand it, the material should be removed from the trailing end of the trailing shoe. We all accept that leading shoes are more efficient, and because they do more of the work, the lining on that shoe tends to wear more rapidly. The turning of the wheel has a servo effect on that shoe, conversely it has an opposite effect on the trailing shoe - producing an opposing force to the effort applied by the brake cam. This force prevents the cam turning and applying full effort to the leading shoe. Removing some material will reduce the opposing force and also increase the wear rate on the trailing lining so that it matches the wear on the leading lining - thus giving better braking and more balanced lining wear. Of course, a certain amount of trial and error will be needed to find the optimum amount to remove.
More specific to our postwar machines, the fact that Mr Douglas used mild steel brake drums does not help - the coefficient of friction would have been much higher had cast iron drums been used.
Why didn't manufacturers do it? Firstly, in the relatively traffic free days of the 50's, it was probably not deemed necessary. Also, it meant that 2 types of shoe would have to be made (leading and trailing) - with the possibility that the amateur mechanic could fit them the wrong way round with disastrous effects (this, of course, still applies today!!!).
