What is this old thing ?

Some old Walbro pump. Somewhere you may have the hose that plugs into that outlet. This offers fuel to filters, pails or jugs for other boats.
I'm glad to see that red storage plug. It helps lots.
As on my Bertram, It may have a filter before the pump. If it's a strange looking filter, it is, I have the part numbers.

Both MANs? Were these the factory original engines?

Id guess that it's been used for the Racors as well as to fill the secondaries, hence the quick disconnect for different applications. Fire it up and confirm the function. Useful device...

I learned very early with Cat C15’s to never use that mechanical pump as after every use it ALWAYS leaked air. A remote electric pump upstream was the reliable outcome.

My experience was that once I first cracked the handle open and pumped, I was doomed. Happened a few times, and then I replaced them yet again and never used them. Problem solved. I installed an electric priming pump and incorporated a ball valve bypass ahead of the check valve at the head of the return line. That enabled me to pump fuel into the secondaries and beyond, let the flow run for a few minutes, and then close the bypass valve to set a nominal pressure in the line prior to shutting down the pump. Always fired with that arrangement, and I never again messed with the manuals. It was several sets of pumps, not just one. Maybe I was buying out of a bad batch from the local dealer...but it was frustrating to beat the band.

Personally I would never own a boat without electric priming pumps again.

Whether it s a brain fart causing you to run a tank dry, a filter change or another issue, they will save the day and for just $250 per engine. No brainer.

the manual priming pumps are pretty useless except maybe to push the last bit of air out of the lines but not for a filter change.

I have a pretty big Oberdorfer priming pump for each MAN engine. It's very convenient during filter changes. I haven't used the manual pump yet. I'll see if I have any pics or will take some next time I'm at the boat.

I actually extended my manifold that serves the four engines and added two pumps to it. One pump can serve as a priming pump to push fuel to the filters, etc. The second is a larger pump with a dedicated Racor and can be used to polish fuel or also transfer fuel. We tend to run on a shared main tank and transfer reserves to the main. This keeps the fuel fresh throughout the boat while also allowing us to clearly monitor remaining range in a manual type manner to back up the gauges via the ships log.

Really try to keep things simple here. We want to avoid changing valves on long runs. Tired minds make silly mistakes.

Yes, my large pump for polish and transfer is an Oberdorfer with a bronze vein pump. I have tried to keep all pumps aboard with a shared model of motor and capacitor.