Welcome to the new and improved Proflex / K2 Tech Forum!
the `gusseted` down tube has an extra plate welded underneath where the downtube meets the headtube for added strength, the slotted seat tube ( thats the big hole infront of the swingarm ) was where the brake cable originally went through when the bikes were fitted with center pull cantilever brakes. as for the 959 stickers i think someone else might be able to explain that.
Please excuse my ignorance but what does "no gusset on downtube but with slotted seat tube" mean in newbie terms?