What is the possessive noun in the sentence, "Baseball" rules were written in 1845."?

As written, none of the words are possessive . However, if we change the word "baseball" to "baseball's", then "baseball's" is a possessive noun.

A possessive noun is:

• a noun (person, place, or thing), and
• possessive (indicating an ownwership relationship to another noun)

In the sentence Baseball rules were written in 1845, there isn't a word that indicates possession:

• rules is a noun
• baseball is an adjective and tells us what kind of rules
• were written is the verb
• in 1845 is the preposition that tells us when the rules were written

If however, we write the sentence this way: Baseball's rules were written in 1845, now we have a possessive noun - "baseball's".