A feeler gauge is a tool for measuring gaps, or clearance between two parts. This one is an old, all-brass one.
The entire tool is only 3 1/8 inches long and 1 1/4 inches wide. It could easily fit into one’s pocket. There are 25 blades, numbered from the thickest, #8, to the thinnest, #32. We are assuming that this is an “imperial” feeler gauge—measuring in thousands of an inch.