Below, a couple of "Unbreakable" brand clamps:
The smaller clamp is stamped "G.M. Co."
Below, a stamped metal square made by the same firm:
This company is not, as you would suspect, General Motors, but a manufacturer that made a variety of tools, as shown in the 1951 Popular Mechanics ad below, and in the pics below it. That's the extent of the information I can unearth on the company.
|The Family Handyman, May 1955|
Note the unusual button for selecting the direction in which the drive turns:
Other offerings pictured on the web: