As far as the 273/318 goes - As I say, I think they're the same. . . .
Certainly after stripping the 318ci down it appeared, visually the same as the 273ci.
The 273ci was obviously a lot less popular, so I'd struggle to believe it was made differently from all the others . . . . .
The stroke and bore would probably give it away . . . . 318ci could be a stroked 273ci