Except how easy back then, especially if an African-American was charged, was it to get a room full of cops (and the notary would verify, because he'd be in on it)to say, " he confessed freely..." and the police's word was taken as gospel. Confessions could have been beaten out of people, or at the threat of a gun to the head, and no evidence to the contrary. Even today, you see things like the Stacy Bond case in Ottawa, Canada, and this was caught on videotape.
To clarify: I'm not saying there were no abuses of the system in the old days. I'm only saying that it was not a foregone conclusion that a "signed confession" -- signed when there were no witnesses to testify to the circumstances -- would mean Bea Carroll was doomed to be convicted of murder after Superman had handed that "confession" to the governor.
It occurs to me that, offhand, I'm far from sure on just what the law might have to say (in the 1930s, or more recently) about a person wanting to "revoke" a signed confession, or what "appeared to be" a signed confession.
I have the vague impression that this has been done, successfully, upon occasion, but I don't know what the process is or how much it takes to persuade a judge that the confession should not be introduced before the jury, during a criminal trial, as "valid evidence."