Your answers are fonts with serifs.
The lighting on the dog tag may make the letters appear to have serifs, but they could perhaps actually be a sans serif with the light hitting some sides of the raised lettering more than others, which could give the impression of serifs.
If you have the original and not just a photo, close examination with a magnifying glass should resolve this point.
don

I knew I had seen these tags before. This was the answer I gave then:

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Stamped tags? If so probably not made from a conventional font.
Good matches for appearance are two fonts based on engraving systems. Lineto's Gravur Condensed http://lineto.com/The+Fonts/Font+Categories/Text+Fonts/Gravur+Condensed/ and Optimo's Gravostyle Basic http://optimo.ch/typefaces_Gravostyle-Basic_all_CharacterMap.html Only major difference that I spotted is the open top 4 and the tails on 6 & 9. There are other engraving/machine plotting fonts, but they are generally wider than the letters on the tags, particularly for O, C etc. which usually are rounded without the parallel stroke section on the sides.
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http://typophile.com/node/115178
Don

This has come up before: node 32896

Also, using only "military dogtag font" in Google turns up quite a few interesting hits.

Comparing the present image with the one in node 3286 I conclude that both are monospaced fonts, which makes sense for stamping keys.
But I see significant differences, even with this ambiguous image, in letterforms.
Look for example at the center strokes in _E_ and _F_. Much further in this tag than the one in node 3286. Huge serifs on _I_ in node 3286, but none detectable in this image. A letter by letter comparison reveals other differences.
Based on this image I still don't see serifs, or at the most I could perhaps see the hint of tiny ones.
For a substitute you could use either a Sans serif, as suggested in previous posts, or a monospace version of a tiny square serif like Cushing Two http://www.myfonts.com/fonts/bergsland/cushing-two/ or Bushing http://www.myfonts.com/fonts/bergsland/bushing/ or Knubi http://www.verg.com.au/portfolio/knubi/
Unfortunately, the more I look at this I am, persuaded that there is no close digital font.
Don

Don

If all else fails, there are always some monospace sans-serif typewriter fonts such as Orator http://www.myfonts.com/fonts/linotype/orator/ to consider.
Don