Just picked up a military 5600 and its great. I don't think the readability on this reverse display is as bad as its been made out to be.
Anyway this is the strange mark printed on the very bottom right of the glass lense. This is just below the word CHRONO. Its a dash followed by a zero and another dash.
Any ideas, here is a link to a pic with the same marking. It must be on all the DW5600MS-1 units, but what is it?