The title of The Door into Summer
was triggered by a remark which Heinlein's wife Virginia made when their cat refused to leave the house: "He's looking for a door into summer." (See first link.)
But other than the page Robert A. Heinlein Quotes About Cats on A-Z Quotes, I have not found any mention of Heinlein writing about real-life cats. Did he write or comment on the real-life cats who inspired his cat characters?