I have a '74 Euro (MFI) Carrera with 2.7L engine. My car was one of the first cars built for '74 model year, was actually completed in mid 1973 and registered by the original owner in Sept 1973.
The engine was replaced sometime in the mid-late '70s (don't have the original paperwork on engine replacement) with a blank 7R case. The engine type was left blank (where it would normally be stamped 911/83) on the case but they re-stamped it with the original engine number 664xxxx but font is slightly different. My engine is being rebuilt right now so was able to get a bunch of photos of date castings and such.
The 7R case date stamp is 10 75, so it was cast in March 1975. As djpateman said, many of these magnesium cases never got a date stamp. There was another 7R engine case from a '73 911 up on a stand in the shop last week and the little circle area on that one had no date stamp. I'm not sure if the replacement cases were stamped more frequently than regular production ones or not.
When the engine case was replaced, they rebuilt the engine with the original cylinder heads and most other components. My cylinder heads just came back from the machine shop and you can see the 5/73 date stamp.
