Luckily, I was able to limp home using only my rear brake. Once home, I quickly learned I was lacking the upper tongue/lock washer that stops the bolt attaching the brake stay to the fork lower from backing out under vibration:

I've since bought the washer from classichondarestoration.com and went to install it today. To my dismay, I noticed I was missing another piece; the brake stay collar:

My question is, how important is this piece and what purpose does it serve? Where can I get it? I don't want something like this to happen again. And yes, I have the lower brake stay tongue washer as well. If you have a big brake CL, please check to make sure you aren't missing these on your bike!