1st class vs first class

1st class and first class both are nouns.

Word	Noun	Adjective	Verb	Adverb
1st class	Yes	No	No	No
first class	Yes	No	No	Yes

As nouns, 1st class and first class are synonyms defined as:

1st class and first class: mail that includes letters and postcards and packages sealed against inspection

Other synonyms of 1st class include 1st-class mail, first-class mail.

1st class (noun)	first class (noun)
mail that includes letters and postcards and packages sealed against inspection	the most expensive accommodations on a ship or train or plane
	the highest rank in a classification
	mail that includes letters and postcards and packages sealed against inspection

1st class (adverb)	first class (adverb)
	by first class conveyance; with first class accommodations