Rarity (and desirability) of a slant six/ factory 4 speed?

The use of the term 'standard' is somewhat false when used on a 4 speed but regardless, everyone knows what it means.....a non-automatic trans.

The 3 speed on the tree is still plenty cool, even though it was common. Perhaps the king of all oddballs? The Citroen 5-on-the-tree.