I would like to point out that square grands were still made long after Mozart... up until around the 1860's at least.
Yes, Duo-art is an old brand of pneumatic-player system.
As for the value of the instrument... if it is a square grand the rule of thumb is that as an instrument it isn't worth much.
They make great desks though

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Seriously though, mechanically square are quite inferior to modern pianos. Replacement parts are difficult to find and often need to be fabricatred. Tuning stability due to age and lack of care is often poor.
Another thing and this is probably the most important... pianos are very complex machines. They consist of THOUSANDS of parts under several TONS of tension. They are 85% wood. Largely glued and doweled together. One can not estimate the value of a piano without actually physically inspecting it and then making an evaluation.
Give me the make, and I can tell you how old it is. If you are really interested in finding out the value, contact me privately and I will put you in touch with someone in your area who can appraise it for you.
- R.Cromwell
Cromwell's Piano Service