Campana? Forse intendevi canneggio....quindi una e' L e l'altra ML.
Hai ragione Gigi, anche se poi la campana della ST-308 è leggermente più piccola di quella della ST-307.
Non so se una differenza di 0,33 mm possa differire nella classificazione di un tipo di canneggio.
Comunque, ti riporto testualmente le specifiche tecniche tratte dal catalogo ufficiale presente sul sito di Holton nel 2000:
Holton "MF Horn" ST-307 Specifications
Key: Bb
Bore: 11.89 mm (.468")
Bell: 5.0 diameter yellow brass with
wire-reinforced rim
Mouthpipe: One-piece, yellow brass, specially tapered
Pistons: Monel material with double-slotted spring barrel,
enclosed stainless steel springs and silent two-point
non-metal piston guides
Slides: Main tuning slide is reversed with new round
single-radius crook. Nickel-silver outsides and
brass inside slides, U-pull on 1st slide assembly
and fixed finger-ring with stop on 3rd slide.
Waterkey: Tuning slide and 3rd slide
Finish: Clear baked epoxy lacquer
Mouthpiece: 7C
Case: Deluxe wood shell with dual handles
Holton "MF Horn" ST-308 Specifications
Key: Bb
Bore: 11.66 mm (.459")
Bell: 5.0 diameter yellow brass with
wire-reinforced rim
Mouthpipe: One-piece, yellow brass, specially tapered
Pistons: Monel material with double-slotted spring barrel,
enclosed stainless steel springs and silent two-point
non-metal piston guides
Slides: Main tuning slide is reversed with new round
single-radius crook. Nickel-silver outsides and
brass inside slides, U-pull on 1st slide assembly
and fixed finger-ring with stop on 3rd slide.
Waterkey: Tuning slide and 3rd slide
Finish: Clear baked epoxy lacquer
Mouthpiece: 7C
Case: Deluxe wood shell with dual handles
Per il resto, sono identiche...