Going through " Firearms, Traps, and Tools of the Mountain Men"
Carl P.Russell, 1967, I could not find any specific match of what seems to be a rudimentary poll type axe of notable age, probably 18th century into early 19th. As noted it is likely to be be British or American, and the 'cartouche' is in the typical stamped location on heads of these axes.
On p.408, the author notes :
"...not all historic axes were marked, but more than a few bear names, symbols and signs as well as a variety of decorations. Sometimes it is difficult to identify the smiths touch mark or trade symbols among the miscellania of decorative filled lines, cusps and notches; the mixtures of scratched scrollwork and floral engravings; the stamped diamonds, HEARTS, crescents, crosses, stars and chevrons".
In the book there are many plates of drawings, lists of makers (I could not find the initials IS but it seems such convention was common).
There are lists of names of firms supplying trading goods to the Hudsons Bay. Co. prior to 1821. It is 8 page manuscript found in many American libraries including Western Museum Library, National Park Service, San
Franscisco.
In "Swords and Blades of the American Revolution", George Neumann, 1973,
Page 265 (34a), an axehead remarkably similar to yours is shown as
a SQUARE POLL HATCHET, the size listed compares to yours as well as the shape. It is suggested as c. 1720-1820, which though a broad period, supports the idea of 18th century use as per the character of the corrosion and pitting.
https://www.furtradetomahawks.com/mo...axes---10.html
JUST FOUND THIS.......the link above lists important note, saying the heart shape was common British convention,
A title,"Captain Jones Wormslow: A Historical, Archaeological and Architectural Study on an 18th c. Plantation Site near Savannah, Ga." William Kelso, 2008
A trade axe found:
"...historic records indicated the remains of Ft. Wimberly most likely constructed by Noble Jones an original English settler of Georgia. Records suggested Ft. Wimberly had been constructed on site of Jones earlier fortification, a timber guard house known as Jones Fort buily in 1739-40"
The axe had stamp with W MATHER in full name (rarely seen, usually initials) presumed to be William Mather apprenticed in Sheffield in 1727, still taking his own apprentices in 1739.
As the heart design was apparently favored convention, and Sheffield was sending edged weapons and tools to colonies in these times, I wonder if perhaps an apprentice of Mather going on his own might have used the heart with his own initials?
Another trade axe with a heart design appeared in:
"The Bulletin of the Fort Ticonderaga Museum" Vol.9, p.175, Ft. Ticonderoga Museum, N.Y. 1952, +