Simple..... Pic 1; The captive threaded nut* on the right hand side is sprung back into the hole in the left side - see pic 4. The whole band is now slid and positioned over the nut, barrel. Put the grip on, insert bolt and tighten. Bolt head pulls captive nut down AGAINST the other side of the band. Making it as tight as a proverbial ducks bottom.
These two examples made in an hour. All they need is phosphating.
* captive nut made from suitably BSF threaded, turned down hexagonal nut rivetted into band