A fluid-steel OO 16-gauge, even with it's forend missing, is still a somewhat unusual and desirable item. David is right, take care of it because replacing it won't be easy. The OOs were argueably a bit better made than the Field grade guns, and many more 12s were made than 16s.
Why is it so many Smiths end up missing their forends? You could almost start a cottage industry creating replacements for the ones I've read about here. I've owned and shot several over the years and never had one come off.