The 'lighter' shoes were manufactured from an aluminium alloy with a low Zinc content...Presumably as part of the drive to restrict the consumption of aluminium to more important uses the zinc content was increased, thus increasing the weight...
It's worth noting that when modern linings are bonded to the high zinc castings the temperatures used can cause the casting to 'sag', altering its radius...I think Saftek have addressed this problem with their production method but it's worth checking before having bonded linings applied to the 'heavy' brake shoes...
Rik..Newfren are an Italian company that have manufactured friction materials since the late 40's...I've used their disc pads and they work well...Ian