Present Fort Cochin area before 1947 was part of Bombay Presidency of British India while adjoining Mattancherry was in erstwhile princely state of Cochin.
For residents of Ernakulam area, Cochin for decades meant Mattancherry, but now Cochin or Kochi encompasses all parts including W Island and Fort Cochin.
Princely Travancore and princely Cochin before 1947 had contiguous areas mixed in the present Ernakulam District. For example, Edappally was in princely Travancore State while adjoining Palarivattam was in princely Cochin State. Now Kochi city is in Ernakulam District while Ernakulam is only a component of Kochi city.
No wonder North Indians are amazed and confused. From 1947 to 1956 Travancore and Cochin princely states merged into Thiru Kochi state and in 1957 Malabar also merged to form present Kerala state.