Not 100% sure what you mean,
Some cameras take plate holders that have a lip on the bottom as well as the sides, so it's not a modification, more likely your not using 100% compatible plate holders. That doesn't mean what you have won't work fine.
I don't have a Bergheil but non of my 6 9x12 cameras have anything similar, however I have a boxes with of 100+ metal plate holders sorted by size and then edge fit and while most have a flat bottom edge just a few have an edge strip.
Or are you talking about howe the glass plate fits into the holder ? Photos would be useful.
Ian