These are Walbro FRD-2 low pressure fuel pumps (2.0 - 3.5 psi) marketed by Dickinson for use with their diesel stoves and heaters.
Diesel heaters and stoves are often supplied with fuel via a gravity-feed diesel "day tank" which is refilled, as necessary, from the vessel's main diesel tanks.
A Walbro FRA or other constant rate fuel pump can be used for filling the day tank.
However, it is also possible to supply the stove or heater with diesel fuel by lifting it directly from a main fuel tank to the oil metering valve in the stove.
This 20-000 model pump is suitable for diesel lift applications, when the fuel tank is within approximately 4 vertical feet and 15 horizontal feet of the stove's oil metering valve (carburetor).
For fuel lifts greater than approximately 4, and certainly greater than 6 vertical feet, or 15 horizontal feet, use the more powerful Dickinson #20-002 pump + regulator assembly - which is highly recommended for all diesel stove and heater installations; see Related Products, below.
The higher pressure 20-002 pump + regulator is also recommended for installations in colder climates (Alaska, Canada, Spitzbegen, etc.), where diesel fuel viscosity can increase, making it harder to pump.
Dickinson uses these 20-000 low pressure pumps so as not to overflow the stove's oil metering valve (which happens at 4 psi). The pump will slow to almost stopped when the stove or heater is in use and the pump outflow pressure limit of about 3 psi is reached.
These are "pull" pumps which can lift fuel a fair distance, but cannot push it very far. For this reason, in direct-feed applications, they are typically mounted at about the same level, and close to, the stove's oil metering valve - so they don't have very far to "push" the fuel.
These pumps are only available in 12 volts.