The 7 behind the trademark means the grade of the thread gauge.

A thread gauge is a tool used to measure thread size, and a pass-and-stop gauge is a type of thread gauge. The grade of thread gauge is usually represented by a number, and different grades represent different tolerance ranges and accuracy requirements.

Thread plug gauge is used to measure the accuracy of internal thread size and consists of a pass gauge and a stop gauge. The pass gauge is used for test screwing. If it cannot be screwed in, it means it is unqualified; the stop gauge is used to check the thread length. The qualified thread length should be within 2 pitches. The thread ring rule is used to measure the correctness of the external thread size. When the general gauge and the external thread of the workpiece are screwed through, the stop gauge only partially screws with the external thread of the workpiece, and the amount of screwing does not exceed two pitches, the external thread can be determined. The thread is qualified.