Been out-of-commission for a while due to illness, but my friend AARC called my name, so I'm rousing myself to reply.
First let me say... speaking as a longtime cannonball hunter myself, when I see an ID-request here about an iron ball, I always start with the sincere hope that it will turn out to actually be a cannonball.
Unfortunately, in this case, although as Smokythecat correctly noted it does look very much like a British-made RevWar cannonball, it fails to pass three crucial ID-tests for a cannonball.
1- In the calipers photo, your ball appears to be somewhat egg-shaped... wheras, actual cannonballs were a "true Sphere" (as near-perfectly round as a marble).
2 - The calipers-measurement photo shows your ball is larger than the "nearest size" of actual cannonball, a 3-Pounder caliber. That type's diameter was specified to be 2.84-inches, and the diameter of yours is 2.96-inches. A 3-Pounder cannon's bore diameter was 2.90-inches, thus your 2.96" ball would not fit into that cannon's muzzle.
http://www.civilwarartillery.com/shottables.htm
3- Even if there's .12" of rust/dirt encrustation on your ball, thereby increasing its actual iron diameter, its weight of 3.408 pounds (3 lb. 6 oz.) is significantly greater than the 3.05 pounds (3 lb. 1 oz.) specified for the weight of a 3-Pounder caliber cannonball. (The specified weight is given in the Shot Tables chart at the link above.)
Being significantly overweight is the biggest problem. Here is what I believe is the answer to the conundrum of your ball being only 1/8th-inch larger than a 3-Pounder cannonball, but fully 5 ounces heavier. All actual cannonballs were cast-iron. Steel is a denser alloy of iron than cast-iron, and steel thereby weighs "about" 10% more than the same size of a cast-iron ball. Doing the math:
a 3-Pounder caliber ball weighs 49 ounces,
and "about" 10% of 49 ounces is 5 ounces,
totaling 54 ounces,
which is very-very close to your ball's 55-ounce weight.
So, I believe your ball is made of steel. There were no steel cannonballs. (The sole exception to that rule is a civil war era British Navy 10" Heavy Caliber solid-shot cannonball, made to smash the armor of an ironclad warship.)