Chris, Since no one has tried to help yet, I will try. When you say pin, do you mean at the end of the forend the push button. If it is missing try putting a 3/16" dowel through the hole and hopefully the dowel will go between the clip and spread it, then gently pull down on the forend as the "J" spring will still be holding it. If it doesn't work, keep trying it, the dowel needs to go between that clip.
If that is not the "pin" I don't know what else there is, email me and I will give you my phone number and try to guide you.
You can still get a rough forend from Numrich.
Standard, 12/16 Ga. Model: L.C. SMITH NEW MODEL HAMMERLESS Item No. 851730 Retail Price: $46.90