That it is not the intention of this act [68-1-1, 68-1-2, 68-1-5 N.M. Stat. Ann.] to abrogate any existing mortgage or encumbrance upon timber, or contract for the sale, cutting or logging of timber, and it is therefore provided that any timber included in and described in a bona fide mortgage, encumbrance or contract entered into prior to February 1, 1939 shall be and the same is hereby exempted and excepted from all of the provisions of Section 2, Subparagraph C [68-1-2C N.M. Stat. Ann.], of this act for the term and duration of such mortgage or contract; provided that in any event no trees shall be harvested less than ten inches in diameter outside bark, at a point twelve inches above the ground.