Stumbled across this on Amazon (the description of the book explains its origin):
https://www.amazon.com/dp/B082838YYY“The Pursuit of the Pankera: A Parallel Novel About Parallel Universes” by Robert A. Heinlein
Basically its from an unpublished manuscript of a story he wrote in parallel with his book “The Number of the Beast”.
To be auto-delivered to Kindle pre-purchasers on March 24, 2020.