I remember those. They used to be sold rather widely as novelties on keychains, etc., when I was a boy. Haven't seen any of them in years. IIRC, all of them used "blank" ammo.
FWIW, I've seen a couple of fully functional semiauto pistols which weren't a whole lot larger that were made around the turn of the last century, also in Austria, IIRC. Sold under the trade name "Kolibri", they used one of two miniscule CF cartridges: one 2.7mm and the other 3mm, I believe. Never actually handled one, though. At the prices being asked, I sure as heck didn't want to risk having to buy it if it got inadvertantly damaged in the process.