It's a part that screws to the bottom of my camera creating a quick release to attach the camera to my tripod. I lost the last one before I got my digital camera and then I found it again, but promptly forgot where I found it. A few day ago it occurred to me that there might be a chance of finding a replacement part, even if the tripod was twenty years old. A quick search on Amazon found one for twelve dollars with shipping - that sold me plus there were many comments from people saying they would get multiple ones because they were losing them as well, which made me feel a bit better as well. So now I can take pictures like this (my street at dusk):
I'm very happy right now - I really love how this tripod works - you know when you actually enjoy using something as opposed to just enjoying the results? I'd really like to take some horticultural photos soon. Maybe even some to share!