Will soldering a cast brass finial to 38mm brass handrail tube discolor either metal?
I have silver soldered this size in the past for a large commercial ice maker. I did use an expander to stretch the copper to be a snug fit in the fitting (German ice maker US made compressor); after silver soldering, I cleaned up the pipe and manifold with a light (400 grit) sandpaper then sprayed with a clear sealer since this was used with food production and inspectors look for anything ugly.
Even though this is just a mechanical joint, I would use a swage tool to expand the copper for a snug fit, or your solder connection will be the same as a grub screw.