copyholder vs mechanical device

copyholder and mechanical device both are nouns.

Word	Noun	Adjective	Verb	Adverb
copyholder	Yes	No	No	No
mechanical device	Yes	No	No	No

As nouns, mechanical device is a hypernym of copyholder; that is, mechanical device is a word with a broader meaning than copyholder:

copyholder: mechanical device used in printing; holds the copy for the compositor
mechanical device: mechanism consisting of a device that works on mechanical principles

copyholder (noun)	mechanical device (noun)
mechanical device used in printing; holds the copy for the compositor	mechanism consisting of a device that works on mechanical principles