Villiers wx11

Can any body tell me why some Villiers WX11 Pistons have a cut away on the bottom of the skirt, the reason is to lengthen the port opening time , but which engines required it ?
Regards

usually its the alloy pistons with the cut out for the wx11 lzha or wx11  waha spec engines
witch were the higher rpm engines 1500rpm instead of the usual 1200rpm lighter piston and bigger suck time for the faster speeds requared from them, cast iron pistons were fitted for the standard lz lzh and wah specs ,but for later life and wear and tear the chooses were for oversized bores  alloy or rarly used cast from the  suppliers
hope this helps 
kev

Thanks Kev i was just curious on why villiers would want to lengthen the inlet faze on some engines ??
regards