I spotted this in a case at an antiques shop in central MA and jumped on it just for the sight of it, but have since come to learn it is a rather iconic little tool in the type laying world: a Star printer's make-up rule (or composing rule) used in letterpress printing for measuring and justifying lines of type. Not in the least my expertise, but in the realm of printing history these seem to hold a special place; I read one wonderful story of a print man in his 90s (in 1993) who had carried one just like this around in his pocket every day of his life since his first day on the job in a print shop as a very young man. The STAR  Rule, indeed! 

1 15/16" w x 1 1/2" t. Good antique condition, general wear, some rust/dark spots on the reverse, all of a piece with it.