Well, I think I got to the bottom of this...it gets long so bear with me.
The bike that I bought came from the race shop in Cypress. I knew that when I bought it, but didn't think too much of it. It was a brand new, stock 2014 YZ450 in the crate.
Apparently they get an allotment of bikes that they set up and ship out to their top amateur riders. Before they send them out, they might swap out certain parts for whatever reason. However, this bike was never sent out to anyone, and because I wanted a new 2014 bike (mostly because you could buy them at a substantial discount) the dealer had to hunt around to find one.
When I took the clutch apart, the basket and inner hub were black billet, but had no markings on them of any sort, so I assumed they were OEM. I talked to Yamaha about this, and apparently they get an allotment of clutch parts from Hinson without any markings of any kind and put them into their amateur race bikes, along with the stock pressure plate (which is how this was set up). These parts were brand new, with no notching on the basket or anything.
So, it makes total sense that the stickers that I found in the strainer came off of the back of this clutch basket. Obviously, they should have been removed.
Now we are back-tracking to see if this bike might have had a high-compression piston or different cams put into it at Yamaha, or anything else. It is definitely a fast bike for supposedly being "stock."
This story sounds kind of weird, but that's what I was told and it seems to be confirmed by a couple different people at Yamaha, so I am going with it for now.
I'm just hoping that those stickers weren't affecting oil flow and the valvetrain was getting adequate lubrication. We'll probably pull the head and look for any signs of abnormal wear at this point, just to be sure.