Here is a King mellophone mouthpiece in size 6, silver plate. It's part of the CKB mouthpiece line from Eastlake Ohio. It was designed by the Conn, King, Benge people at the time called UMI: United Musical Instruments.
Interesting info from eBay seller hefner_instruments...
In the 1960s, Zig Kanstul designed the F. E. Olds marching mellophone, with the cornet-wrap leadpipe and tuning slide, as well as a deep-cup trumpet mouthpiece to accompany it. By the 1970s, he designed the King 1120 marching mello [with the Master cornet-esque tuning slide], and he had improved the mouthpiece to what became the "Benge Mello". Because the rim was roughly a Bach 6 size, it became the Benge Mello 6, the basis of the design here.
King/Benge eventually was purchased by Conn in 1983, which in turn was purchased by Skåne-Gripen at Bernhard Muskantor's behest in 1985. The firms were reorganized into United Musical Instruments, and brass mouthpiece production was outsourced to Lausmann of Nauheim, West Germany. The Mello 6 began appearing under the Conn and King banners, with catalog number1216. When Lausmann began making this design with Chuck Ward's heavyweight exterior around 2000, it became the 1736 CKB/UMI Mello 6. The CKB and UMI names are effectively interchangeable, and some versions bear the King brand and don't have the small outside scallop.