Whether an insufficiently stamped instrument is a valid document? Can it be admitted in evidence on payment of penalty ?

Section 35 of the Indian Stamp Act, 1888 stipulates that no instrument chargeable with duty shall be- admitted in evidence for any purpose whatsoever by any person authorised by law (such as judges or commissioners) or by the consent of the parties (such as arbitrators) to record evidence; or shall be acted upon; or registered; or authenticated by any such person as aforesaid or by any public officer, unless such instrument is duly stamped. Provided that any such instrument shall be admitted in evidence on payment of the duty with which the same is chargeable, or, in the case of any instrument insufficiently stamped, of the amount required to make up such duty, together with a penalty of five rupees, or, when ten times the amount of the proper duty or deficient portion thereof exceeds five rupees, of a sum equal to ten times such duty or portion.Further, an insufficiently stamped instrument is not an invalid document and it can be admitted in evidence on payment of penalty. [See K. Narasimha Rao v. Sai Vishnu]