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”.