I had read in various places that the 91mm were aluminum alloy and the 93mm were orig brass and then sometime after '99, nickle silver (as in the '61 Soldier). I do know one thing, a neglected 91mm can "weld" itself shut over time but no matter how stuck/neglected the 93's are (brass or nickle silver)...they can be restored to operation.
Have had several uncleaned TSA 91mm knives that were either "welded" upon arrival and one Champion actually welded shut while waiting to be cleaned in my box, It was fine when I bought it (needed only minor cleaning, all blades moved freely) but It sat about a year in my storage box and now, every blade is frozen in place. Even resorted to trying WD-40 penetrating oil but no luck.