Mp5_Rizzo, thank you for taking the time to take these pics for me. You are exactly right about the Recoil/Action spring. I pulled it and instead of 15", it measured just a tad over 14". I stretched it out to just a tad over 15" and reinstalled it. I doubt that remedy will last long, so I'll be ordering a "new" replacement spring.
Is the knock down pin on your barrel a factory installed pin? I understand that Remington used to sell these pins to the public. I'm thinking they probably included some kind of drawing with the dimensions for the exact location. I have a friend that's a retired machinist, but not a smith. But with the exact location for a 20 gauge, I believe he could install the pin. I'm guessing the actual pins were the same for 12, 20, and 16 gauge barrels.
Again, I want to thank you for your help and apologize for the delay in replying to your post. Thank you very much.