First class and Unpaired

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

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

Unpaired adjective - Being one of a pair or set without a corresponding mate.

First class and unpaired are semantically related. Sometimes you can use "First class" instead an adjective "Unpaired".