If you are thinking about replacing this valve section with a modern 4-valve section, be careful; your valves may all have different bore sizes. Check out Adam Peck's beautiful old Cerveny elsewhere on this list. I believe each valve is a different bore.
My Cerveny Eb has 4 valves, the first two are smaller than the 3rd and 4th.
If you are going to replace the valves individually, get out your checkbook, it'll cost you a bunch. Your old valves are almost certainly worn out though.
I looked at your photos and couldn't tell where the 4th valve had been removed. It is a nice looking old tuba. How's it sound?