I haven’t held this axe/hatchet, so my analysis is conditional.
The blade was made by a smith who bent a piece of flat stock mild steel around a mandrel to form the eye. Touch your thumb & forefinger together to get a pretty good idea of the shape. In the fingertips a strip of high carbon steel, much harder than the flat stock, was inserted to create the edge. The resulting sandwich was then forge welded together.
The harder metal could hold an edge & the mild steel was resilient to take the shock of the blows. The touch mark is an anvil on a three legged stand. ( Turn the photo above to orient the edge downward in its natural position.) A touch mark is stamped into a finished piece by an individual smith or as branding by a forge.
The technique I have described is still in use today. As a general guess, this example dates from the late 19th to early 20th Century.
The ‘S. S. US’ stamped onto the blade could mean a number of things. It could be the property of a steamship; a surplus US gov’t axe owned by Sam Smith or something else. Unless somebody has a reference, it will remain in identified.