Yes. in the 70's sold one by one, later in a service pack (small bag of 5 or 10 units) I forget which.

I had this problem once, I ended up buying two chromed metric acorn nuts, that way both looked the same. The leftover went onto another bike that arrived w/o that fastener.

Those parts do show up on ebay from time to time. but finding an auction for a single caliper w/o brake block holders may be cheaper.