Looks like an antique fuel flow transducer – not sure that it is part of the original PA-28 fuel system but might be rather a retrofit
Some PA 28’s have a fuel pressure gauge.
This is on a 2010 vintage PA28-161, which has an electric primer pump and a fuel pressure gauge.
The connection on the right hand side of the picture goes to the engine side of the fuel filter, and the left hand side goes to the little cylinder primer tube. I assume therefore that it’s a small gear pump for priming but it looks more like a sensor, so who knows.
I would be very surprised if it was a pump. It would be difficult to construct a pump like that, online and with a sealed motor. It could be a flow transducer but on a PA28, and I have never seen such a transducer in GA. I reckon it is a pressure switch.