If any person shown on the last equalized assessment roll to be the owner of land within the proposed district shall be unable to sign by reason of legal disability or shall cease to be the owner of all or any part of such land before the petition or supplemental petition shall have been filed with the clerk, said petition or supplemental petition may be signed by the legal representative (as hereinafter designated) or other successor in interest of said owner. All petitions signed by a person other than said owner shall be accompanied by written evidence, satisfactory to the clerk, that the signer
(a) In the case of a legal representative, is the duly appointed guardian, executor or administrator of the estate of said owner, or
(b) In the case of other successors in interest, is the holder of legal title to, or has the beneficial ownership in, said land.