Denis Wick DW5509 Straight Mute for Bass Trombone or Alto/Tenor Horn (Aluminum). Only used twice. I ended up deciding not to continue with Alto Horn, but held onto this mute long enough to make a final decision on whether or not to ever add Bass Trombone to my arsenal (I will stick with Tenor Trombone).
Denis Wick makes some of the best mutes and mouthpieces in the industry, and although an Englishman, most are made for American designs and shanks. The corks are also very well installed on his mutes compared to some other brands that I have tried. The lightness of this aluminum mute makes it very easy to hold while playing, if you are going for an effect or just a temporary note or two. Otherwise they stay in most horn bells even when playing at ffff.
Although Denis sadly passed away last year, his company survives as he had delegated much of the work to relatives and peers around 2016. The quality has not changed as a result. I bought this one in June 2025. They go for $93 new (although tariffs probably mean that prices are fluctuating wildly at the moment).