Look closely at this illustration of Meyer’s Ochs.
If we color the right hand, we can see that there’s arguably five fingers: red, orange, yellow, green, and blue. Given the difficulty of placing a finger beneath another finger, we can assume that the yellow finger is really just a gap between the second and third finger.
With this assumption in place, we can argue that the third and fourth finger are gripping the sword, the second merely wrapping around the top, and the first finger (blue) is loosely resting on the handle. This forms the so-called “handshake grip”.
Conversely, the back hand is gripping the handle with the first two fingers and the other two are resting on the pommel.
But what of the thumb?
This is where my club has an internal debate. Given how loosely the right hand is, some of us (myself included) argue that the sword is being supported by the right thumb from beneath. This grip, which is seen with one-handed swords held in Prima, makes the sword light in the hand and offers fine point control.
Other members of my club argue that the thumb is alongside the handle, almost inline with the flat. In effect you are pinching the handle between the thumb and palm. While they agree that it offers less point control, they assert that you don’t have to move the thumb as far when executing a cut. (Some repositioning of the thumb is required in both cases.)