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This is a tool for checking the coupler height of Sergent engineering Sharon couplers in HOn3.
The bottom of the tool has notches to fit on the 10.5 mm gauge HOn3 rails.
One side has a hole for checking the installed coupler height by fitting the knuckle inside.
The other side has a platform that conforms to a standard coupler box.
When the platform fits under the car, the coupler box is ready to install.