The site seems to be down now, but Robert Oppelt had a fantastic page where he demonstrated a variety of German bow holds and discussed the benefits of each of them. Maybe someone "borrowed" it and has it saved somewhere? Point is that there are a few mainstream ways to hold a bow, but there are a lot of variations. In almost every variation I have seen, the contact point is your finger tip, or the last joint.
Starting with your little finger, and then working "up" to your thumb:
Little finger: Generally speaking, most people have the tip of their little finger on the ferrule. From what I gather from Don's photos, his is mostly on the tip of the finger. Depending on the size of your hand, you might have the tip, pad, or side of your finger on it, but I can't think of a "school" where that isn't the contact for your little finger.
Ring finger: There is a bit of variety here. Some people have it touching the top of the ferrule or a little farther back like in Don's "from above" picture. Some people have it more or less floating where it would touch the frog if the throat wasn't cut out, and some have it up next to the middle finger on the "back" of the frog.
Middle finger: Every school I have ever seen has this finger under the stick, keeping the bow horizontal/holding it up against gravity. It is usually where the stick and the frog meet. Some people have it very curved and touching on the finger tip, others have it far less so, and it is more on the pad of the finger.
Index finger: This finger is either under the bow the way Don has it in his photo, or slightly on top. This is one of the most common deviations from school to school, but basically the "on top" method is where the side of the tip of your finger is on the corner facet of the octagon, and it can apply some of the weight into the string.
Thumb: A lot of variety. Some people have it on the opposite corner facet from their index finger, some have it directly on top. Some have it pointing down the stick, some across. Different schools have it across the bow touching the index finger, and you will even see it almost in a "thumbs up" position. Generally speaking it is also responsible for helping you get weight into the string, but you don't want it to be locked and white knuckled.
With that said, my grievances with Steve's variety of grips/holds are as followed:
The little and ring fingers should be as above, because you are sacrificing a lot mobility/flexibility in your hand, especially with your ring finger looped through the throat that far. It may feel like you have a better "grip" on the bow, but it also means that the bow is stuck and can't go anywhere. Your hand needs to be loose and responding to the bow change initiated in your back, and it looks like you are creating a lot of unwanted/needed tension. Loosening up will make bow changes a lot smoother and faster passages easier.
I like your middle finger in the first picture the most. Again, I cannot make my finger wrap around the stick/frog as much as the third picture without my finger being locked/tense.
Your index finger in the second picture looks like it is between where Don has it "under" the stick, and I would have it on the top corner of the octagon. Somewhere in between the first two (leaning towards the second) would be where I would have it, but some would say it should be even more under the stick. The other two photos lead to a tenser finger, and depending on how tall your frog is can also mean opening your hand too wide. You also lose mobility when it's on the joint/second section of your finger.
The thumb is a little hard to see. I would lean towards your second photo again.
I think Don's photos are a good place to start. I have a smaller than some/most hand, so any photos that I post would not necessarily be that helpful as my own personal variations are a little bit different still. If I had a better camera/was more technically inclined, I would document the variety of holds I am familiar with. I should really develop some better computer skills...
