Not seen one like that, a tiny inline 3 cylinder/piston type like that. The ones I have seen are either radial or axial piston hydraulic types. Many with variable delivery rates.
It is possible someone cobbled it together from a small inline pump by changing the valves.
What is it installed in, the boat mfgr?
Boat model?
What type of boat, dedicated pleasure or a fishboat conversion?
Have you looked all over the parts, pump, cylinder, etc.? Maybe there is a label somewhere although they can be upside down so you may need a mirror and flashlight to find them. If a name or serial/model number is stamper or embossed into the casing maybe it can be seen that way.
If you have a boatyard or older chandlery nearby that has been around a while maybe they have an idea.
I have no idea but any info. like I have asked about may trigger an idea from someone.
Any other photos that show more other than just the crankcase?
Last but not least is what is it doing or not doing that prompts the question?