there is no difference ! That czech ammo that comes in crates was not designed for the Sks specifically haha!! Its a 7.62x39 stripper clip !
Its not designed for just one rifle. I use that czech ammo and i have no probems loading it into my cz858 or my sks.
I always get a kick when I go to the gun store and hear people refering to those crates as "sks ammo"! Its 7.63x39 , its not just made for the sks !
Hope this helps you out, but this is at best just my opinion!