Lever up or down, changes the top to toe relationship of the cam to the pads, the forward shoe is the leading shoe, if the lever is down , when it is pulled forward the toe of the cam operates against the leading shoe, since it is at a minor diameter compared to the top it gives a slightly more sensitive feel, .
Conversely with the lever up the top of the cam at the major diameter is pushing the leading shoe.
None of this matters much, what really matters with the back brake is the amount of pad material on each shoe,
the leading forward shoe has a reduced surface area, the leading edge is cut back at least an inch compared to the trailing shoe, dont mix these up,( this isnt mentioned at all in the Manual
, my bike still has its original shoes and one definitely has more material than the other, I think its covered in the Rupert Ratio book) if you get them relined make sure you follow the original layout, or you will have a grabby brake with no feel.
My 71 has the lever down, my pals later bonnie had the lever up the brakes felt about the same.
To remove the brake springs/ shoes lift/ pull out and up against springs a bit, the end of one shoe out of position and upwards so it is no longer located, springs will now be slack, refit the same way.