Why is there a big hole in this tsuba?

[Tim Evans]

Occasionally we will see images of tsuba with the seppadai area cut out. Various speculations have been offered as to why, however, I recently came across these images showing that this was done to mount a metal tsuba on a bokuto. There is attendant documentation implying that this bokuto dates to about 1888, or 20th year of Meiji. The tsuba looks much older. Mounting old tsuba on bokuto seems to have been a martial arts fad at one time based on on the number of modified tsuba that turn up.