Proof of the execution of an instrument which has not been acknowledged may be made by: (1) all of the parties who executed it or any one of them; (2) a subscribing witness; or (3) other witnesses in cases mentioned in 1-5-302.
Who May Prove Execution Of Instrument
Mont. Code Ann. § 1-5-301, under Proof of Execution.
Mont. Code Ann. § 1-5-301