The Starrett 657S snug has two 1/4" (6.3 mm) holes to adapt Starrett 196, 650, and 651 series dial test indicators and 657Y indicator attachment for use with the Starrett 657X rod (not included),. A knurled thumb screw provides secure grip when making adjustments.

Test indicator gauges, also called gages, are precision instruments that measure short linear distances. They are commonly used for detecting deviations from tolerance limits, such as small variations in flatness or height of an inspected workpiece, and permit measurement in narrow grooves or small bores. A test indicator gauge consists of a ball-tipped contact point on a levered arm, also called a measuring insert, which pivots from a hinge point. The reading indicates the degree of angular displacement of the arm. Test indicator gauges are commonly used in manufacturing, machining, and mechanical engineering. Standard test indicators measure the displacement of the arm perpendicular to the axis of the indicator. Dial and electronic indicators are very similar to test indicators, except that the axis of measurement is parallel to the axis of the indicator. Test indicators can be analog, with a mechanical dial, or electronic, with a digital display. Some electronic models transfer the data electronically to a computer for recording and potential manipulation.

The L.S. Starrett Company manufactures precision measuring tools, metrology and testing equipment, and saw blade products. The company, founded in 1880, is headquartered in Athol, MA.