This guy might know:
http://mvdataplates.com
I did a websearch for "US military ID plate font" and came up with a LOT of pictures, nothing on which specific font should be used (san serif, times roman, or pica size, nothing).
There's likely to be a printer's user's group or website where, if you posted up a photo of and ID plate with height/width (use a caliper) of the letter, someone will take up the challenge and tell you what font is used. A knew a guy years ago in LA who ran a print shop. He once said something about prospective employees needing to know at least 100 different font types before he'd consider hiring them.
You figure there'd be SOMEwhere on DoD websites stating this explicitly, but I don't know how to get there from here.
Hope at least some of this helps. I'm curious too.
Norm