First thought was that it looks d.i.y or cottage industry standard.
Some people believe magnets can cure fuel
problems but 2-3 litres seems big for something like that. Never seen one myself.
A small compass
should tell you if there's a magnet inside though.
2-3 litres makes a header or accumulator tank more likely.
tend to be mounted as high as possible .
Accumulators often have a Schrader (tyre) valve one end, easy enough to feel if there's one at the top.
Don't forget to let us know will you? Hate that