Sorry Graeme,
The plug with the oil indicator is the one to the left of the mag pinion (you can see the rod going into the top of the vertical oil gallery). The blanking plug fills the hole for the cable of the optional exhaust lifter fitted to some of the early 750's.
Manfred,
The oiling system on the S6/T6 is very simple - the oil pump pushes the oil up the vertical oil gallery, from where some is fed to the 2 cam spindles (via the brass 'Y' piece), the remainder is then fed to the timing side main bearing bush and into the crankshaft to lubricate the big ends. Any excess continues up the vertical gallery to lift the indicator rod and then flows out of a spill port in the side of the gallery (to return to the sump)(the weight of the indicator rod controls the oil pressure!). The rest of the engine is just 'splash' lubricated.
Regards,
Eddie.