Shimano does have all of their parts diagrams online, so you could compare the parts diagrams and parts numbers. Just search google for part description followed by "PDF".

http://si.shimano.com/php/download.p...6800-3607A.pdf

I've rebuilt a pair of SPD pedals. They are quite easy. Just pull the spindle with the plastic tool.

Disassemble. Grease, reassemble, and adjust cones. Slide the spindle back in.

Shimano uses loose ball bearings which you could replace too.

Oh, I didn't realize one could buy extra-long axles. Interesting option for some riders. Anyway, the diagram lists 3/32" bearings. I see 24 per pedal on the diagram, so I'm not sure why they list 62 (each pedal, or pair?). The one I rebuilt did have a bunch of little tiny bearings though, so perhaps 62 is the correct number.

Comparing the diagrams for the PD-6800 and PD-M520, the 6800 appears to have a longer sleeker spindle, and no place for the 15mm pedal wrench. But, who knows, they still may interchange.