I think it's just a reflection. That flyer always bothers me because of the location of the bumper caps (they don't match the awarded skill shot that way lol). Another interesting thing about this image is that the yellow inlane guide has yellow post sleeves instead of the double rubber rings that ended up shipping on the game. I switched mine to Cliffy yellow post sleeves, and it looks a lot better imo.

I've seen a picture of one somewhere that had either ball bearings or pachinko balls. I wouldn't fill it up all the way due to the weight.

Oh, here it is: http://pinside.com/pinball/forum/topic/pdi-glass-my-opinion