letter box vs box

letter box and box both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter box	Yes	No	No	No
box	Yes	No	Yes	No

As nouns, box is a hypernym of letter box; that is, box is a word with a broader meaning than letter box:

letter box (noun)	box (noun)
a private box for delivery of mail	a blow with the hand (usually on the ear)
public box for deposit of mail	a (usually rectangular) container; may have a lid
a numbered compartment in a post office where mail is put to be called for	separate partitioned area in a public place for a few people
	private area in a theater or grandstand where a small group can watch the performance
	the driver's seat on a coach
	any one of several designated areas on a ball field where the batter or catcher or coaches are positioned
	evergreen shrubs or small trees
	the quantity contained in a box
	a rectangular drawing
	a predicament from which a skillful or graceful escape is impossible