All sorts of instruments have been made and engraved with competitors and other sellers names on them, over the years. This looks like a 50 - 60 year old Edgware Rd. (Besson/Boosey & Hawkes - London) tuba that Buffet must have contracted for (perhaps in the 1950s or 1960s), so they would have a tuba to offer in their brochure, at that given point in time.
Leblanc was making some 3+1 Besson-ish looking tubas (albeit very small bore: .625”) at that time, but I’m sure that Buffet didn’t want to buy them from them (arch-competitor), yet probably wanted to offer a competing instrument, thus contacting Boosey and Hawkes.
Over the years, we've adjusted/repaired/repadded quite a few Conn-made "Shooting Stars"-era alto saxophones with "Evette" engraved on them, as well as some Italian-made "Grassi" tenor saxophones. (In the very distant past, there WERE some "genuinely Evette and Schaeffer" alto and tenor saxophones.) ...and we've all seen curious collaborations, between Yamaha and C.G. Conn, Schilke, G. Leblanc, and Bach. (etc., etc., etc.), and even Conn-and-Olds, Blessing-and-Conn, and others.
The above scenario makes the most sense to me.
The pictures are not high resolution, but I can’t make them into anything other than an old English-made Besson tuba.
bottom bow/bell/everything else: In my estimation, the cost of the repairs would quickly exceed the value of the instrument.