Here are some pictures after repairing and cleaning it up a bit.

The leak has been fixed. It seems to play fairly well. I'm struggling with some intonation issues but that might be just me. I may need to cut some tuning slides. The 4th valve seems to be a 2nd valve used as a 4th. I also need to add a water key or spit valve to the main tuning slide.

I am the new owner of this tuba!
It is indeed a C tuba. When it arrived it had a leak between the 3rd and 4th valve that made it unplayable. Maybe that's why the seller thought it was in Db?