Search this site for past threads on the Alvar Aalto Model 60 stool, there is a whole bunch of information on these stools and how to determine authenticity.
The wood plies at the bend in the legs is the first characteristic to check. After that, dimensions need to be checked.
As I said, take a look at the other threads here first, and if you still have concerns post at the end of one of those threads. doing it that way will notify the people on those threads that there is another question, and elicit the best response for you.
Recently I've encountered with 3 four-legged chairs which were labelled "Alvar Aalto, Artek" by a seller.
I've made measurements, inspected the seat and the legs and I've draw a conclusion that it's just IKEA type very, very old chairs.
The Aalto seat should be empty like a drum, as a hollow woodf rame is sandwitched between 2 sheets of plywood.
The legs were made of plywood, the real Alvar Aalto chair has a leg made of whole wood. In Artek they take a piece of wood, saw several slits and then bend the leg (see the picture).
And, finally, the measurement didn't correspond to Aalto's chair.
I also posted the same image from a few days ago from another question about Aalto stools.
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