If the said sideplates were lost and were replaced with ones from another Smith he would most likely have had to hand fit them. Also the original serial number will be stamped on the inside of each "original" sideplate. If there is no serial number or the number stamped appears to be of a different style than the rest of the gun I would be very suspicous. Best to have the gun examined by a competent gunsmith upon recieving it back from the current one.