Notice the "Late Marelli" Twin-Plug distributor http://www.pbase.com/slidevalve911rsr/911rsr_factory_parts_marelli_tp_distributor
as used on the 2.8 liter 911 RSR racing engines.
Late in 1973, Porsche released the 3.0 liter RSR and an "upgraded kit" to bring the 2.8 liter engines to the 3.0 spec.
I'm not sure if the Marelli twin-plug distributors were also upgraded to the BOSCH units http://www.pbase.com/slidevalve911rsr/911rsr_factory_parts_bosch_tp_distributor as well.