The R3 is an example of a pneumatic governor pump and was produced for use in the famed W198 300SL Gullwings. The R3 seen here is a 6-plunger model which provided improved efficiency and allowed adjustments to individual plungers and, in effect, injectors compared to previous 2-plunger models. Its inline design and layout also carried over to later fuel injection pump development as seen in the W113 pumps from the 60s and 70s such as the R11, R18, and R20 series. 

There are a number of obvious differences between the R3 and later pumps: Various ports near the plungers that remain as vestigial remnants on the W113 pumps are accessible and usable here; additional oil inlets and outlets in the bottom center remain separate from the plunger housing; the governor has a completely different setup including a diaphragm and spring system for controlling air/fuel mixture; lastly black oxide coating remains rather than zinc oxide that eventually became standard.