According to the website, that is an optional flow adjustment valve. https://www.fluidotech.it/en/products/t ... no-series/

Took my pump apart today and found hairline cracks running through on 3 sides, I'd guess that's why it's not getting up to pressure! I think my current plan is to use the fittings from my current pump on the new ones so I don't have to figure out a way to loop the new valve in.