If, before January 1, 1981, a parent or other person has in a signed writing appointed a person to serve as the guardian of the person or estate or both of a minor, or as the guardian of the property the minor receives from or by designation of the person making the appointment, such appointment shall be deemed to be a nomination of a guardian if the requirements of Section 1500 or 1501 are satisfied and, in such case, shall be given the same effect it would have under Section 1500 or 1501, as the case may be, if made on or after January 1, 1981. This section applies whether or not the signed writing is a will or deed so long as the person signing the writing had at the time the writing was signed sufficient capacity to form an intelligent preference.