Post-Office box vs letter box

Post-Office box and letter box both are nouns.

Word	Noun	Adjective	Verb	Adverb
Post-Office box	Yes	No	No	No
letter box	Yes	No	No	No

As nouns, Post-Office box and letter box are synonyms defined as:

Post-Office box and letter box: a numbered compartment in a post office where mail is put to be called for

Other synonyms of Post-Office box include call box, PO Box, POB.

Post-Office box (noun)	letter box (noun)
a numbered compartment in a post office where mail is put to be called for	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