I have seen an antique toy gun for sale with three much smaller versions of this, with brass cases 3 holes in the base and a lead bullet. The rounds I saw were about the size of .32 S&W Short. I looked at them thinking they were original rounds, then saw the 3 holes in the base and thought they could serve no purpose other than toy imitations, so must have been made for that reason.
This toy gun was an old die cast cap gun (made to look like a Luger) in its original box, with spaces in the vacuum formed plastic packaging for the rounds and a die cast Sheriff’s badge (with a Luger?). There was no way to chamber the rounds in the toy gun as play ammo, so I presume they were carred by the kid as “spare rounds”.