The DW5521 piccolo trumpet mute fits the Bb and A bells, and the DW5520 D-trumpet mute fits the G bell very nicely. I am highly pleased with both mutes!
I hope my information will be helpful, in case you ever receive questions from other individuals regarding the proper mute for a G-trumpet. Of course, there's a possibility that the D-trumpet mute may not fit all makes and models of G-trumpets, but it certainly fits the Kanstul 1520, if not others. (By the way, my G-bell is a 4.5-inch bell (about 11-12cm) - I don't know if that is a common size for a G-bell or not.)
Thanks for your helpful information and your excellent craftsmanship!
Russell Thompson