It shall be the duty of the register of deeds on receiving any conveyance or instrument affecting realty, including construction liens, to cause such conveyance or instrument to be entered upon the numerical index immediately after filing if such conveyance or instrument contains or has an exhibit attached containing the full legal description of the realty affected. Instruments purporting to release, assign, or amend a conveyance or instrument previously recorded shall contain the book and page number or microfilm or computer reference of the previously recorded instrument and a full legal description of the realty affected.