Marlin Firearms Collectors Association

I'm new to the forum and have a question about my Model 81. The cartridge guide spring is broken and I can't figure out how to replace it. Numrich gun parts has the original style in stock. I can't find a set screw or pin (so far) to release the old one so I can get it out. Any thoughts are appreciated. My grandfather won this 81 in a raffel when I was too small to pick it up. It's the gun he taught me to shoot with and I want to pass it on to my grandson. Sure like to get it back in "fighting trim".

Thanks in advance.
Cliff