Fuel pump information needed

[Crash66]

I purchased a 1997 Chaparral 240 Signature. Engine has been replaced and I cannot locate the serial# in the usual places. So... Unknown. It is a Volvo Penta 5.7GL.
I am having stalling issues at slow rpms. It is very intermittent. The fuel pump assembly has been replaced by an aftermarket, maybe even automotive, pump. I wanted to replace the original but the part number I get #21608511 for the assembly has a low pressure side and a high pressure side.??? My shop manual shows a different assembly that is NLA. It only mentions a low side.
Can someone explain? Just low pressure, high pressure. Why both on the pump assembly?
Thanks for the knowledge.

 

[alldodge]

Your serial number for a VP is the model, but its more then GL, should also have an A thru F.

The fuel pump number should be 3858261 (item 1)
https://www.marinepartseurope.com/en/volvo-penta-explodedview-7744290-23-6922.aspx

The entire GL series is here
https://www.marinepartseurope.com/en/volvo-penta-cat-7744290.aspx

 

[Scott Danforth]

if your in the US, the website is volvopentastore.com.

however before I would go replacing parts, I would diagnose the problem.