any thoughts,ideas ? ID this sofa  

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Yeah the slots are likely for ganging sofas together or attaching other modular pieces.
I don't know who made it, but it's sharp all right-

Not much help really, just thinking out loud
The base looks the same as the Florence Knoll sofa, but the top is different. Did Knoll ever reuse the base for an armless sofa? Or else an armless copy/homage by an alternative manufacturer?
(Not much help really, just thinking out loud)

Steelcase
possibly
Definitely looks like contract- sectional

I was thinking
steelcase as well. The shots of the bottom and the base look the part

thanks for your...
thanks for your thoughts.
my first thought,when i saw it, only a front picture,on the sellers site,was Florence Knoll too,but i think it isn't , it looks like stand in a hotel lobby or
lawyer/architect `s office.I search every day but doesn`t found a similar sofa.
Thanks for more loud thoughts.......

pure coincidence brings light
pure coincidence brings light in the dark, today i could ID my leather sofa. 🙂
It`s Model Constanze,designed by Johannes Spalt for Wittmann KG / Austria in 1960 / 61.
Found it as daybed and chair,(the chair looks exactly like my sofa),
but does`nt found a pic of a 2 seater.

Congrats
On the ID, and thanks for posting the update.

here is
here is a pic of two chairs and a matching table

yawzaa
and thanks for letting us know! I always dig these long ago, but not forgotten, ID efforts.