whats the difference in the original hybrids and the MK-2 ?

i have bought ,25 FX hybrid slugs that had MK-2 on the bottom right hand corner of the box.
if you go to PA and search .25 hybrids, you will see a picture of the box.
notice where that slug is....thats where it says mk-2.
yes i know you dont see mk-2 on any other boxes.
but, trust me mine said mk-2's !
thats why i have questioned about the difference.
 
here ya go....yes i know the box dont say MKII but mine did !