The magazine well looked like a Mosin to me, and then on the Mosin page I spotted the graphic that I posted. I think it looks like #6 in the image I posted, but since I don't read cryllic, I really don't know what they have identified it as.
Looking at it now, though, I have noticed that I missed a very critical detail. The locking lugs on a Mosin are horizontal. The lugs in that graphic are vertical, so it's not a Mosin.
Until I read more, I'm guessing it is a Gewehr 88, or a Mannlicher rifle.
As shots are fired the clip remains in place until the last round is chambered, at which point it drops through a hole in the bottom of the rifle. This system was used in almost all Mannlicher designs and derivatives, and while it allows for speedy reloading, it also creates an entry point for dirt.
The G88 uses these en bloc clips:
