The '73S had Biral cylinders and a Magnesium case. It also had MFI, not CIS.
I believe the '73 Carrera 2.7 was the first time they used Nikasil.
The cylinder heads for the CIS were quite a bit different than the E or S engines, and had the injector port built in.
To figure out what engine you have, take a look at the right side of the engine by the fan housing to see what engine type you have.
This is from my 73.5T CIS, type 911/91. The T engines came with cast iron cylinders
