I'm guessing it is part of an unloader, looks like part is missing.
Are there spring loaded diaphragms on top of the cylinder heads? That is probably where the 1/4" line went. When the pressure is up to set point, the unloaders open the intake valves on the compressor to stop the pumping.
If it's a gas or diesel engine, that's how it works instead of stopping and starting the engine.
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