Fake Kartell?  

They were never stamped as...
They were never stamped as such, the branding was part of the injection moulding.
They were made under licence (at least here anyway) so it wouldn't surprise me if either it was omitted by accident, or for some reason the agreement was dissolved and someone did a naughty and filled in/ground out the branding.
Or its a copy, I have no idea 🙂

Kartell marks
louie_louie,
The early pieces produced under license in the United States were marked with both Kartell and Beylerian. Other parts of the world may have different marking for the licensee.
I have seen Gino Colombini waste baskets and Simon Fussel Stacking Drawer Programs without marks and assumed (same as Heath) they were made after the licensing agreement had expired and were "in the style of" and of no value to me.
Best of luck

Fake (probably)
These cabinets are widely faked - see link below. I wouldn't touch one unless it had the embossed Kartell marks underneath. Originals are neither expensive or rare so it really beats me why anyone would want a repro
http://authenticdesign.com.au/real-vs-replica-the-anna-castelli-componib...

it depends
it all depends on where and how old the piece is. vintage 1960's and 1970's kartell (before the age of globalization in the 1990's)actually sourced their manufacturer locally where the products will be sold following their high level of manufacturing standards instead of shipping them from italy and all over the world.
i have the universale chairs by joe colombo and they were marked 'beylerian limited usa' and the magazine holders one made in south africa and the other form mexico, i also have some of the trash/planter bins with no kartell marks but with paper labels instead.
the type of plastic used will also determine its age and if it is a vintage piece or a recent production. it is not the plastic itself that makes kartell products expensive, it is the quality of the tooling molds used to manufacture the items. the smoother the metal molds, the better the finish plastic products will be not requiring a lot of unnecessary treatment.