I've watched a number of the factory videos on You Tube showing assembly of a double, e.g., Heym. Looks like they use what I'll call a 'shoe lump' system and solder the barrels directly onto it. In other words, the bottom part of a 'monoblock'. Just watching the process suggests to me they don't use an especially high heat brazing material. Some of the heavier silver brazing material I have requires red hot heat to flow. If my observations are correct, seems threading is nice but not necessary. Threading would, I think, making replacing a barrel easier. Been kind of wrestling with this for my own project.