Some more photos of old shave gear I found online, this time brushes from around the time of the US Civil War. Three things that strikes me right off the bat is the use of string or twine to secure the knot, the length of knot in relation to the diameter, and the narrow neck between the knot and the handle.
While the use of a string to secure the knot most likely was due to the lack of modern glues, and the extreme length of the knot can be down to either the hair used or just the style of the times… the narrow neck probably had a practical purpose; to stop water and lather running down the handle.
Perhaps the length of the handle was only used in a mug, and the notch allowed the lather application grip to be closer to the tip. It would mate with the notch between thumb and index finger. You can see the modern brush within this design, if you imagine the knot part sawed off separately.