Starting with a batch of GDX-VPG in October 2024, we are using that marking to indicate that the sensor was a “give away” to visiting teachers. In the case of the GDX-VPG, these photogates are perfectly good for most uses, but the acceleration they will report when used with a Picket Fence will differ, depending on the direction you drop the Picket Fence through. They will give a good acceleration in direction shown by the arrow on the GDX-VPG.

I think we should use this general convention to make “give away” items, so that a person receiving the gift does not return it and expect it to be repaired or replaced. The “X” should be near the connector for power or near where the cord connects to the sensor in the case of a wired sensor.