For holding, through grommets, reverb tank to steel chassis, so not the usual small ones for DSP boards etc. You cannot reinsert, blind, as the
4 splay bits are permanently splayed out. Find a metal plate with holes the size to push each rivet in, to close back the 4 splays, then old soldering iron and hotmelt glue the 4 together. Reinsert through grommets and chassis, then the closer rivet, with more force than usual to break the hotmelt .