I have seen two different approaches to setting square staples.
I don't know if it's a key difference but you have to remember original barrels were iron and today's barrels are steel so an old technique may not work the same on a new product (not a "metal head" so don't know one way or the other).
You can still drill a round hole, set the square leg in and then peen the hole so that it appears square (pretty much the same as you would do with a modern round staple).
Alternately, I have seen originals where it was clear that the (hole) was chiseled in, most likely with a cape chissel, leaving a slightly raised piece of metal at the hole edge.
The staple was then set in and the raised edge is peened/swedged back into place trapping the staple - similar to what you would do with a raised dovetail.
Sorry, no pics to post. I use modern staples and the setting tool commonly available from the builders supply places.
