"The Bosch ones lock in quite nicely without loosening up, even though they are not brass."
True, they are a bayonet fitting, a pain to make. Years ago I had a 1/2" Bosch and made an adaptor to take the threaded brass guides. All the guides that I now make for my two Makitas are steel which are held in place with two screws, such a sensible method, I never cease to be amazed that all makers haven't copied Makita and Hitachi.
Harry,...will everyone on the forum please recount stories of when and how many times they have damaged cutters on steel guides and how often steel nuts have worked loose whilst routing, who knows, I may have to change my mind, I'm always receptive to new ideas when a convincing argument is presented.