POB vs letter box

POB and letter box both are nouns.

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

As nouns, POB and letter box are synonyms defined as:

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

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

POB (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