Interesting. The stamping 'AT' would indicate you have a Factory replacement case for the original case from a '74 Carrera 2,7. Check out this thread from the Early 911 registry. Does the stamping on your engine look like the second photo in the tread?
http://www.early911sregistry.org/forum/showthread.php?t=5806&highlight=case
Also, does the engine have the type # stamped on it? It should be type 911/83.