The mounts are named for the seven classical planets, and that naming is the clearest fingerprint of where the modern system comes from. Reading the hand and reading the sky were fused in the chiromancy of the later medieval and early modern period, and the planetary vocabulary entered from astrology rather than from any independent tradition of hand reading. Where a system's parts are borrowed, the borrowing is usually visible in what it calls them.
The Mount of Saturn is anatomically the pad at the base of the middle finger, over the head of the third metacarpal. Its prominence is a matter of the soft tissue over the joint and of how the hand is habitually used.
Two things follow. Mount prominence is strongly affected by build, by handedness and by manual labour, none of which the chiromantic reading isolates, so a reader judging a mount is judging a physique as much as anything else. And the planetary names carry astrological meanings in with them: what a mount is said to signify tends to be what its planet signifies, which means the mount system inherits its content rather than deriving it. The atlas sets out the tradition's own reading elsewhere on this page. Here it records what the structure actually is and where the name came from.
Sources Introductiones apotelesmaticae elegantes in chyromantiam, physiognomiam, astrologiam naturalem, complexiones hominum, naturas planetarumJohannes ab Indagine (Johannes Rosenbach) with Kaplan's Functional and Surgical Anatomy of the HandEmanuel B. Kaplan, 3rd edition revised by Morton Spinner