How to make a "shooter's sandwich"

A photoessay at The Guardian provides step-by-step instruction on this Edwardian creation.  Pictured above are the ingredients (" You'll need your choice of crusty loaf, a couple of good steaks - these are rib-eyes - roughly the same shape in plan as the loaf, 500g of mushrooms and 200g of shallots") and the final product (" Serve sliced like cake accompanied by something vaguely vegetable-based to assuage the guilt.")

Perhaps someone from across the pond can tell me why it's called a "shooter's" sandwich.*

Addendum:  Just found a long discussion thread of this sandwich at BoingBoing.

* and Arlene found the answer in this article that accompanied the photoessay: "A triumph of Edwardian cuisine, the shooter's sandwich was originally created as a snack that Cook could make the night before it was required, effectively making a beef Wellington portable so a gentleman needn't get peckish while hunting."