An alloy body means it's a 9/10 speed freehub so that's why your 8-speed lockring doesn't fit. You shouldn't use 8-speed cassettes on that hub because the sprockets have shallow splines that don't go all the way down to the bottom of the valleys on the freehub body. Doing so will ruin the alloy freehub in no time.
A picture would help a lot but I'm pretty certain you have a 9/10 speed hub there.
Mark.