Decode Differential Markings

that is exactly what a lot of detail restorers do.

It looks like a smeared rubber stamped 8Y2 probably BY2. Two people with the initials BY's so he got a 2 added to his stamp? I am not sure what the one above it is though?

here are some examples of inspection marks. there is a similar type on a pictured differential.
http://www.mmcdetroit.com/ICCA/References/Inspection_Marks/

specifically look this one it has the other weird mark also. So I would say part of the stamp edge
http://www.mmcdetroit.com/ICCA/References/Inspection_Marks/image_23.html