First class and Preexistent

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

Preexistent and first class are semantically related. In some cases you can use "Preexistent" instead an adjective phrase "First class".

Preexistent adjective - Existing previously or before something.

First class and preexistent are semantically related. Sometimes you can use "First class" instead an adjective "Preexistent".