Well, the interesting twist is that while Quint wears an army jacket, he did not serve in the army - he was navy (hence his experience on the USS Indianapolis). There is no logic to him having an M51 army jacket, other than he bought it for himself as army surplus. The stencil is not military issue either, so he must have gone and made that for himself too (which is a rather oddball thing to do!) But, I guess it all adds to the complex and quirky personality that makes his character!