The county commissioners may at their discretion procure such plans, other than township plans, of properties within their counties, either originals or copies, as they deem for the interest of their counties to have preserved on the files of the registry of deeds. This section shall not be construed to allow the purchase of any plan which the proprietor of any estate is required by law to file with the register of deeds.
33 M.R.S. § 661