John;
Being an American born 1938 of course all I know on this subject is what I have gleaned from reading over the years. What I have gathered here is the more or less standard black powder load for the nominally 2" gun was 3 Drams black with 1 1/8 oz shot. The early switch to smokeless was predominately with bulk smokeless which used the same volume of powder @ about half the actual weight. This gave a very slight reduction in recoil due to the lighter weight of the powder but not really enough to make a significant difference in the gun's weight. With the drop of the shot weight to 1 1/16oz & the increasing popularity of dense smokeless powders which again cut the weight of the powder approximately in half a significant reduction of gun weight could be accomplished without an increase in recoil. This was I highly suspect the reason for the continuing popularity o the lighter load. While it did prove to be adequate for its intended use, there is no evidence I am aware of that it was actually a "Better" load from an efficiency standpoint due to it being "Square".
I agree with all you say above, but would add that my understanding was that as the loads became 'over square' (i.e. longer than the diameter) - the risks of pellet deformation (this in the days before plastic wads) both from wall contact and crushing/cold welding due to pressure increased. Hence, 'long' loads were more likely to have fliers and show irregularities in pattern. Its obviously not an exact rule, but I can see the logic of having a rule of thumb - as they say! 'Square' seems like a good point to start, though I have no idea if say 1.2:1 would be just as good. I suspect by 1.5:1, the pattern might well be suffering.
