As to third parties without actual notice or knowledge to the contrary, the indebtedness secured by any recorded mortgage, or reservation of vendor's lien, either in deed of conveyance or note, bond or contract of purchase covering real estate 20 years past due according to the original maturity date, or some new date fixed by a duly recorded extension agreement, shall be conclusively presumed to have been paid unless the record of such mortgage or lien shows a credit by the mortgagee, or lienee, or owner of the debt, or his assignee of one or more payments upon such indebtedness within the 20 years last passed. And if the final maturity date of the debt is not shown by the mortgage or lien, or a recorded extension agreement, or cannot be determined by calculation from the recitals contained therein, then the time shall run from the date of the mortgage or lien.

The notation of credits mentioned in the next preceding paragraph may be made upon the record of any such mortgage, or vendor's lien, by the owner of the indebtedness, or someone holding a duly recorded power of attorney authorizing it. Such notation must show the date of the payment made, the date the entry was made upon the record, and be witnessed by the probate judge of the county, or his duly authorized clerk.

Nothing in this section shall be interpreted as changing existing laws as to the rights of parties, or their devisees, or heirs-at-law to such mortgagees and lienholders as among themselves.

This section shall not apply to mortgages and deeds of trust executed by corporations and political subdivisions securing bonds and in which the maturity dates of the respective series of bonds which are permitted to be issued thereunder are not fixed in the mortgage or deed of trust.