I'm going to take a wild guess, and say it could be "Prov. Tool Co.", short for Providence Tool Company. PTC held a U.S. Government contract for swords during the CW.

To confirm this, look for the inspector initials of J.M, D.F.C. or R.P.B.

This company produced, I think, only cavalry model swords, and perhaps only in 1862. They proviced 10,434 to the U.S. Army.

One other thought, a relative of the owner of the Ames sword company owned Ames Shovel and Tool Company (see below). I wonder if there may have been a subcontract, and those subcontracted swords marked "Ames Tool Co."? Just another wild guess, as this is completely outside my area of knowledge.

"Charles R. Ames was originally from Massachusetts and built the home on land granted for his grandfather John Ames' service in the Revolutionary War. From John Ames' blacksmith shop grew the Ames company, which began making shovels in 1774. Many of those shovels were used by American soldiers to dig entrenchments at the battle of Bunker Hill in 1775. The company, part of Ames-True Temper, still makes lawn and garden tools. According to the 1983 Ames Co. newsletter, Charles Ames was one of four brothers. Another brother, Cyrus Ames, also settled in the area. One brother, Oakes Ames, remained in Massachusetts with the foundry business which later became the Ames Co. of Parkersburg. Another brother moved to Iowa, where Ames, Iowa, is named for him, the newsletter says. In 1931, Baldwin Tool Works merged with Ames Shovel and Tool Co. of Easton, Mass., to form Ames-Baldwin-Wyoming Shovel Co., headquartered in Parkersburg."