Found: 4-A Rack Ends By a frame builder, for other frame builders
Small parts can often become a pain in the rear for frame builders. They either have to source them, or simply make them. Spencer from Traffic Cycle Design had this issue when it came time to build a custom bike rack for a customer. He turned out a small batch of high quality rack ends manually on his lathe and mill. These can either be welded or brazed into place. Since then, he has looked into production of the 304 stainless steel part at a US based CNC mill. To do this, he reckons $500 worth of pre-sales will fund the tooling and design needed for the CNC mill to make the first production run. If interested, visit his Indiegogo campaign here.