"If no oil flows from the check bore, unscrew the filter and add engine oil up to the check bore.".
This phrase applies to pneumatic governed pumps, used on all MB diesel motors till 1974 and 240Ds till 76 (in the US). The 300D in 1975 started the use of the mechanically governed pump which made it to the 240D with the introduction of the 123 body (1977 in the US).
The mechanically governed pumps are fed oil from the engine internally and do not need any oil maintenance other than oil changes.
Bosch Master, ASE Master, L1
33 years MB technician