Ok, I ran a borescope through the horn from the leadpipe as well as the main tuning slide of both horns and came to some conclusions.
1. Dan was right.
2. For the silver horn, I was mistaken about swapping 3rd valve to the 1st casing and the ports lining up, but not lining up in the 3rd casing. The inlet bugle port doesn't line up with the bad valve, but the outlet bugle port does. When I was using the mirror, I was viewing through the 2nd valve casing ports so I was looking at 1st valve outlet (ok no matter which valve) and 3rd valve inlet (misaligned with bad valve). I was also mistaken on 1st valve not lining up in 3rd casing. I must have got them mixed up when I laid them down. Having a toddler in the room makes it hard to keep track of what's going on.
3. The brass horn's 1st and 3rd valves (known good, serial matching) interchange between themselves, and the silver horn's 1st and 3rd casings. Everything lines up from any position and direction no matter how they are mixed. King 2341 models do have identical 1st and 3rd valves
4. After closer inspection, the silver horn's 3rd valve is not original, but the others are. The clue was in how the bottom of the valve was made, honing marks vs none visible on the others, and the fact that it was perfectly clean beneath the felt at the stem. I'm guessing it is for a King sousaphone since it appears to be a new part and the inlet port is clocked differently.
Thanks for the guidance. I'll be dropping my horn off at the shop in the morning.
Attached pic of offending valve.
