Brad
Many suppliers manufacture instruments in different brand names. As an example, I order a line of wind instruments under one of our brand names.
All of my guitars, amps and most other instruments I sell including pianos are also private lable instruments manufactured by select companies with one of the private lable names imprinted on them.
That being said, it is possible that the tuba that you have is simply that and little will be found out about it.
There is a slight possibility that Besson may have carried a Mastertone brand at one time so you could always email them and inquire. I did a product search for the name on their web site for Mastertone but came up empty-handed.
How old of an instrument do you believe this tuba to be?
For determining what key the tuba is in, you will need someone to play a specific note on the tuba (open, not pressing any of the valves) and find the corresponding note on the piano.
This is difficult for me to explain here but it will likely be the first and easiest note for the person to get a sound out of on the tuba.
Malcolm