What started as a project to get some nice photos of the Salt Shaker mute in the bell of various horns has turned into a great demo of why removable corks on mutes (my system in particular) is such an awesome idea (pats self on back
), so I thought Id do this short pictorial writeup. 
The trombone in question is a vintage (late 1930s ish) Buescher 400 "True Tone." Its a nice looking instrument with some neat art deco accents. This particular trombone has a surprisingly large bell throat, and readily swallows the Salt Shaker with the size 1.5 corks that I install on the mute as a general/initial sizing. In this case, the player would likely elect for a thicker size to position the mute further out in the bell. Size 3 shown below positions the mute much further out. Ultimately size 2 or 2.5 may be a good choice depending on the players desired sound. Size 4 positions the mute quite a ways out to illustrate the range of adjustment that is possible. Size 3.5 - 5 are generally used for Bass Trombones and Large Bore Tenors with very large bell flares.
Bell throat profile of the Buescher 400 "True Tone" :
Salt Shaker position in the bell with size 1.5, 3, and 4 corks.
If you regularly used this trombone along with others that have tighter bell flares, you could simply remove the corks and install thicker or thinner corks to suit the other horns. Quick and easy, no gluing/filing/sanding required. This system also works great with straight mutes, cup mutes, and really any mute that does not require the annular space to be completely covered/closed off.
I thought this might be illuminating/interesting to some of you! As always I am here if you have questions about my designs, through PM or email [email protected]
