Stewart Banjos began making tenor banjos in the early 1900's. It is possible that the one you have was manufactured by another company based on one of their models. Unless it is one of the more elaborate ones, it's unlikely that it would be worth much money. The composition of the head itself is not necessarily an indication of age, and the bridge may, indeed, have been scarfed from a real Stewart. If you periodically check E-Bay (search for Stewart Banjo) you should see a number of tenor banjos going for relatively modest prices, $100 to $200.