Can anyone identify this maker's mark please?

My 4 Eric Saarinen type chairs have some age, but are in generally good condition.
They are very heavy, metal swivel base, thick plastic seat.
They are marked with a capital A and I would love to know more about them.
Thank you for your help.

That should have read Eero, not Eric, apologies.

Yes Designed by Eero Saarinen 1955/56.
Yours are the Tulip model 151.

Thank you for your prompt reply.
Would you be able to tell me which firm produced this copy of the iconic Tulip chairs, I can't find any information about the 'A' mark. Wish they were original.

My book from 1997, just says designed for Knoll associates, New York 1956 to present.

Interesting, so even though 'Knoll' is not stamped on the back they were still produced by the broad company. My next task is to establish what they might be worth as I have to rehome them. Thank you again.