Yes, the original papers indicate it was in lisp. "Why AM appears to work" suggested that a lot of the reason math was a good domain for discovery was just that lisp is well-suited to the domain. And Eurisko could modify heuristics expressed as s-expressions.

People have asked him, but I'm not aware of Lenat ever commenting on the source code. Cycorp is substantially funded by DARPA, and I wouldn't be surprised if Eurisko is part of its IP now.

Feel free to email me to take this offline.