The Acorn denotes RWS manufacture, and probably for a variety of “.22”/6mm single shot “Rifles” with smooth bores, and labelled “6mm Glatt”…the general design is the same as .22/6mm “Boys Rifles”. (Bolt action, half moon extractor, simple stock and fixed sights.). There were also Trapdoor and “Rolling Block” types of actions as well. The 9mm RF version is also known, though mostly in a Warnant trapdoor action.
Also known as “Garten-Buchse” or “Garten-flinte” (Garden guns), suitable for taking small vermin in gardens.
The seemingly “Bottle-neck” design is to allow the shell to fit any chamber of the .22/6mm type ( Short, Long etc) as the paper tube will go into the lead of the rifling.
Fiocchi still makes the 9mm RF version.