I just recently replaced my brake master cylinder on my 1954 3100 truck, and know exactly what part you are talking about.
I couldn't find a replacement anywhere and would like to buy one.

I bought new copper washers and re-used the old banjo bolt.

Attachments
20190117_184929.jpg (308.23 KB, 91 downloads)
banjo bolt

1954 Chev 3100 Truck - Mostly all "Original"
https://lh3.googleusercontent.com/4...7X26uneH2rb3222fNY4s7FvnKH=w1697-h954-no