I took a look inside and noticed the square is in fact created by the light baffle which was confirmed by moving it up and down. The strange circle is actually a silhouette of the piece of metal underneath the negative carrier and above the light baffles. I have no idea what the purpose of that round metal piece is for however. It's held in place by several screws so I'm guessing it serves a purpose somehow. I'm going to see if I can locate the mirror and figure whether it has been dislodged.