These are Walbro FRD-4 higher pressure fuel pumps (6.0 - 9.0 psi) - with an added pressure regulator - 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 heavy duty Dickinson 20-002 model is especially suitable when the fuel tank is more than 4 - 6 vertical feet, or 15 horizontal feet, from the stove's oil metering valve (carburetor).
For the relatively small premium in price, Dickinson recommends this heavy duty pump for all installations of their diesel heaters and stoves as it affords fuel flow pressure adjustment, should that prove necessary in your particular installation; and it has the power to pull fuel from distant tanks if you find you are at the limits of their less powerful 20-000 pump. See Related Products, below
Furthermore, this heavy duty pump is recommended when the heater or stove is used in cold climates - where the diesel fuel can become thicker and harder to pump.
Dickinson uses these higher pressure 20-002 pumps to get the extra lift required to pull the fuel from a distant tank to the pump, but then adds a regulator to limit the outflow pressure to 3 psi so as not to overflow the stove's oil metering valve (which can happen at 4 psi). The pump has three internal check valves to prevent the fuel from falling back to the tank.
The pump will slow to almost stopped when the stove or heater is in use and the regulator outflow pressure limit is reached.
These are "pull" pumps which can lift fuel a fair distance, but cannot push it very far. For this reason, they are typically mounted higher than the fuel tank and within 12" of 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.