https://link.springer.com/article/10.1007%2FBF01934993 https://link.springer.com/article/10.1007%2FBF01934993 which is one of Flajolet's early papers on the top

https://link.springer.com/article/10.1007%2FBF01934993 https://link.springer.com/article/10.1007%2FBF01934993 which is one of Flajolet's early papers on the topic opens with "Approximate counting is an algorithm proposed by R. Morris". My guess would be Morris wrote it out of engineering need, and Flajolet and Martin followed up with formal analysis and resulting improvements.