pillar box vs letter box

pillar box and letter box both are nouns.

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

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

Other hypernyms of pillar box include mailbox, postbox.

pillar box (noun)	letter box (noun)
a red pillar-shaped letter box	a private box for delivery of mail
	public box for deposit of mail
	a numbered compartment in a post office where mail is put to be called for