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.

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. Note that this slow pumping produces a clicking noise which some find objectionable. In these cases, a gravity-fed day tank is preferable.

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 vertical inches, or less, of the stove's oil metering valve - so they don't have very far to "push" the fuel.