SDCL § 18-4-19: Circumstances permitting proof of instrument by handwriting.

The execution of an instrument may be established by proof of the handwriting of the party and of a subscribing witness, if there is one, in the following cases:

(1) When the parties and all the subscribing witnesses are dead;

(2) When the parties and all the subscribing witnesses are nonresidents of the state;

(3) When the place of their residence is unknown to the party desiring the proof and cannot be ascertained by the exercise of due diligence;

(4) When the subscribing witness conceals himself or cannot be found by the officer by the exercise of due diligence in attempting to serve the subpoena or attachment; or

(5) In case of the continued failure or refusal of the witness to testify for the space of one hour after his appearance.