I have a Remington Model 121c that I have taken apart. Now, I cannot figure out how to install the cartridge retainer. I have watched six or eight disassembly videos on the Internet, including one very detailed disassembly video that lasted almost 1 1/2 hours. Not one video mentioned or showed the cartridge retainer. The detailed video showed pretty much every major component from every angle, but there was no shot of the cartridge retainer. It's like it does not exist. When I took my 12c apart, the cartridge retainer fell out, but not so with any video I watched. Can someone tell me how this part goes back in? A photo would be good.