There is some confusion about which rock is the actual Saddle, with two different outcrops being cited within close proximity of each other. The first location, at SX 5409 7441, is the lesser known, but actual rock; a large natural boulder with the inscription "SB" Sampford Bounds.
It is Dave Brewer who leads us to this location, with 'Saddle Rock' appearing on pages 172-73 of the 2002 book 'Dartmoor Boundary Markers'. The ensuing discussion on the whereabouts of the stone is very lengthy and is attributed to the Tithe Commissioners. The name of the Rock apparently dates to 1845.
The second location and, it has to be said, the one that Ken Ringwood's book wrongly identifies, is a part of the barbed enclosure walls of Vixen Tor at SX 5415 7436, and sits partly on Private Land. A reason for the mistake may lie with it having a Sampford boundstone beside the wall.