Paul, 
You're absolutely correct on that 45 percent circumference cut-out being
an anti squeal device. I remember receiving a factory service bulletin
in late MGA/MGB times, (around 1963) that strongly stressed the correct
orientation. There was also a later one that warned of the dangers of
"mix-and-match". Lockheed made the "improvement" to save money on that
one machining step, but the notches work better / are quieter than the
shims. I always just made a quick pass with the mill to "fix" the
notchless pistons when I had them out for seal replacement. That took an
extra 5 minutes and my customers appreciated the peace and quiet.  Jarl
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KILE, PAUL D wrote:
> 
> Paul H.,
> 
> I believe the original purpose of the cutouts was to prevent brake
> squeal.  If so, it would make sense to me to orient the cutout toward
> the trailing edge of the pad.  This would have the effect of "toeing in"
 
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