I'd try calling the library ( http://library.caltech.edu/about/contactus.php http://library.caltech.edu/about/contactus.php ), giving the subject and date infor

I'd try calling the library ( http://library.caltech.edu/about/contactus.php http://library.caltech.edu/about/contactus.php ), giving the subject and date information, and asking where you might start looking. You are going to be fighting an uphill battle against the odds (probably any recording has been lost or destroyed), but it is within the realm of probability.