What is this thing?

[Ukiwis]

I have a small leak out from this device on the pool filler line.

I have no idea what this is or what purpose it serves but the piece of copper has a tiny hole in it and there's a stream of water coming from it.
The black cover to the right in the third picture used to be on it but has disintegrated. It's after the PRV and I'm wondering if it might be something to further drop the water pressure?

Can someone please enlighten me on what it is and what it does?

 

[cwswifty]

To me it looks like the mechanicals that are inside of a pressure gage.

FAQ: How Does a Mechanical Gauge Work?

Marshall Instruments, Inc. is a manufacturer of bimetal thermometers.

www.marshallinstruments.com

 

[Ukiwis]

You're 100% correct. It was a pressure gauge. I think the brand is hayward.