What device must be part of the equipment at inspection stations for measuring tire tread depth?

The tire tread depth gauge is an essential tool used in inspection stations to measure the depth of tire treads. This device ensures that tires meet the legal requirements for safe operation on the road, as insufficient tread depth can lead to reduced traction, especially in wet conditions, and increase the likelihood of accidents.

By using a tire tread depth gauge, inspectors can accurately assess whether tires are safe for use based on the specific regulations in Missouri. The gauge typically has a clear scale allowing inspectors to measure tread depth down to the millimeter, which is crucial for maintaining the safety standards of vehicles on the road.

The other tools mentioned are not suited for this purpose. A punch is more commonly used for making holes in materials, a window tint meter measures the darkness of vehicle window tints, and a string and nut does not provide any method for accurately gauging tire tread depth. Thus, the tire tread depth gauge is the only appropriate device for this task.