Mont. Code Ann. § 25-31-512

If the plaintiff attaches to the complaint or files with the justice at the time of issuing the summons a copy of the promissory note, bill of exchange, other obligation for the payment of money, or other instrument upon which the action is brought, the defendant is considered to admit the genuineness of all the signatures to the instrument unless the defendant specifically denies the genuineness in the answer under oath. In case of a counterclaim, the same rule applies to the plaintiff.