This Besson mouthpiece is a bit of a mystery.  It has a trumpet shank, a deep cup close to Vincent Bach 5V, slightly small throat, and would appear to be for marching mellophone.  However, Besson never seemed to have made that kind other than a Kanstul-esque horn, resembling Zig Kanstul's King 1120 design with Besson/Benge-type casings & trim.  The first valve casing appears to have the Besson Kanstul stamp, and that horn would have been made in the USA, not at Besson in England.  

The Besson Made-in-England mouthpiece's exterior design corresponds to the type from the 1960s-1970s or so, and there is no size marking.

The piece fit okay in a couple trumpet receivers, and true to its design, it does make those trumpets mellower.  The throat seems a little tight for this type, and the longer shank means intonation [especially higher register] won't be as precise as one might want - likely flat and difficult to lip into tune.  

Interestingly, the backbore is tighter than the Yamaha MP-14F4 but the V-funnel cup shape is more like the Yamaha 14F4 flugelhorn cup.

The shank taper seems standard and the end diameter is close to that of the UMI CKB Mello 6, but Besson were no strangers to using proprietary tapers, and the phenomenon of receiver wear means we can't guarantee fit.  

The Besson shank opening outer diameter is 0.3842" versus the UMI 0.3847" for reference.  

The shank opening is round, and the excellent silver plating only has a few persistent tarnish spots and some light nicks/microscratches.

Sold as-is, no returns, pay within 3 days.  US shipping free - caveat emptor as we cannot take responsibility for any mishandling/loss on the part of the courier at any point - this is FOB shipping point.  By buying, you [the buyer] agree to the potential risk of the package not arriving due to courier mishap - be aware of these conditions.

Usually we get the item in the mail within 1-2 days.

Please note shipping delays may still occur.  It could take ~an extra week in some cases.