Sorry to have to say, it is definitely not a Military insignia. I've seen various specimens of those large (1.5-to-2-inches tall) solid-cast brass letters, having 1/2"-long thin pin/spike on the back, for attachment to whatever they were mounted on. As DC Matt mentioned, the pin/spike may have been for nailing into wood ...or for attaching to thick leather. Somebody told me they've been found on very-old telegraph/telephone poles, and somebody else recently suggested here in the What-Is-It forum that these large solid-cast brass letters were from antique horse-harness leather.