Hi Robert...Yes, sorry, it is the Black Pit of Dr. M. I highly, highly recc. I have Black Pit, The Witch's Mirror, Curse of The Crying Woman, The Vampire Collection, Brainiac, The Man & The Monster & The Living Coffin. Here's a link with trailers:
http://www.casanegrafilms.com/casanegra_catalog.html
I was wary too, not familiar w/ Mexican horror, I think they're all wonderful, dripping with everything we love. The Undying Monster? Ill keep my eyes open for it. There's an amazing German film called Der Nackte und der Satan ("The Nude and the Devil," aka The Head, West Germany, 1959). Horst Frank plays Dr. Ood. Need I say more? Best, Chris