I pulled the side off my 300 Bayou today to have a look at my clutches before I put a new top end on it and happened to notice a screen at the bottom of the motor. I didn't realize there was even one there until now. Here is what I found. There was also a dime sized piece of foil from the top of an oil jug.
I don't feel comfortable using that bottom end any more after seeing all the big shiny pieces of metal.

 

I just checked out my spare bottom end and the screen is 100% clean.

 

Damn! I reckon... I thought the screens were located "behind" the crankcase oil drain plug...