Hey Jon!
The Convertible Elite is model U4107, and dates from 1977-80. You can kinda get its vintage by the serial number, they started numbering them at 0001001, so the lower your number the earlier in the run your machine is.
Thats a nice vacuum, excellent carpet cleaner, and definately worth saving.
The Singer C-6 is a rare one, and definately worth keeping, those are rare birds indeed. I have a C-7 here. I'm guessing that C-6 would date from 1963-64 maybe?
The 3rd vacuum is definately a rebuild, as the model 65 Convertible was pink, and did not have the flower power bag. Still its a 2-position Convertible and if it runs good its an excellent carpet cleaner. Model 65 was the first Convertible, and ran from 1957-1959.
Hope this helps some, you've got some keepers there for sure!
Jeff