Caution: I'm gonna ramble
The old style and new style goosenecks should fit pretty much any King sousa, old or new. We had new style and old style goosenecks on our HN White and King Instruments King sousas back in high school. UMI was formed in 1985. The King sousas we had in high school started appearing in the band photos around the 1983-1984 season, so they were pre-UMI Kings, but definitely post-HN White. Sometime later, the band director picked up a couple extra HN White King sousas, including a King Giant. Before the King sousas, he had lacquered convertible/marching tubas; maybe Meinl-Westons.
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I have an old HN White King raincatcher, and the old style and new style goosenecks both fit. Same with the King sousas I played back in high school. As for the changes made to the King sousas over the decades, I actually prefer the bracing (especially around the gooseneck/upper mouthpipe) on the
post-HN White sousas because the bracing is more robust -- I like and prefer the thicker bracing, round flanges, and half-circle brace around the upper mouthpipe better. The HN White sousas
look a little more 'elegant', though. I have not played the newest Conn-Selmer King sousas with the detachable guts.
For reference, oldest-to-newest: HN White -> King Instruments -> UMI -> Conn-Selmer.