I'd try calling the library ( 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.
Comments
Any possibility that the exchange was recorded in some way?
Easy to forgot how recent it is that any talk or lecture is likely to have been video recorded and already available on the web somewhere.
I'd try calling the library ( 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.