Are antique stock certificates that are endorsed/signed on the back automatically "bearer certificates" and of value without proof of ownership?

I have spent hours researching this topic, with no luck. My father (a retired antique dealer) has tasked me with figuring out whether a collection of old stock certificates have any negotiable value.

But before I research the individual certificates, I need to know whether he could even legally claim any possible $ the stocks might be worth. According to pretty much every source I’ve read, you need proof of ownership or family connection to claim and endorse a certificate. But from what I can tell all those sources were referring to certificates that were never signed/endorsed by the original owner (which is typically the case when people find old stock certificates).

But what if the person who owned the stock already SIGNED/ENDORESED IT on the back? That’s the question I can’t find a definitive answer to. Of the many old stock certificates my father got in various auctions and other lots of old paper and antiques over several decades, a couple dozen were actually endorsed on the back and never canceled.

Even though he is not a relative of the original (long-deceased) owners, does the fact that the stock certificates are already endorsed make them automatically "bearer certificates," and my father eligible for any funds they might be worth?

I would really appreciate any help figuring out how to answer this question!