'In the box' is a noun phrase; the noun is box.

An appositive is a phrase, usually a noun phrase, that renames another phrase or noun. A noun phrase is a group of words taking the job of a noun in a sentence. Noun phrases consist of the main noun and any modifiers.

Memory is a noun so that can be used as a noun. Of and in are prepositional words and form a preposition when used in a phrase. So while there is a noun in the phrase, the phrase cannot be used as a noun.

The antecedent is the noun, the noun phrase, or the pronoun that a pronoun replaces.