roman type vs roman letters

roman type and roman letters both are nouns.

Word	Noun	Adjective	Verb	Adverb
roman type	Yes	No	No	No
roman letters	Yes	No	No	No

As nouns, roman type and roman letters are synonyms defined as:

roman type and roman letters: a typeface used in ancient Roman inscriptions

Other synonyms of roman type include roman, roman print.

roman type (noun)	roman letters (noun)
a typeface used in ancient Roman inscriptions	a typeface used in ancient Roman inscriptions