You have the dipstick on the back of the case above the PTO right?
All the prints I look at do not show a hole there, just a raised casting.
Is it a blind hole or does it go into the case?
Stick a piece of wire in the hole if it only goes a short distance then it's a blind hole.
Kubota typically just puts a push in plug over blind holes.
And it's not uncommon for there to be holes that were never used.
And also common for there to be matching holes in the sheet metal that were not used.