The b16a and b18 sleeves are made out of the same stuff. Don't know what the composition is exactly, outer sleeve is aluminum inner is some sort of slightly ductile iron. The B20 sleeve is a type of FRM but not the same as the preludes.
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Don't bother giving me "rep", I won't return it and I think it is stupid.
Gracie Barra Clearwater
Gyms in Clearwater, St. Pete and Port Richey
pm me if you are interested in trying a free class at any of the gyms.