I think that confirms that your piano is rare and all original.
To me,an "art case piano" is one that is not of the traditional spade leg or squared ferrule leg,music rack,lyre,molding,hand carved ? etc. One that is more contempoirary in it's look.The traditional Steinway 380 sketch case(spade leg,modern music rack,lyre)style was initiated around 1900 depending on the model Steinway. I find there were always exceptions to the rule as in a hand made piano.Of course,this is just semantics. The modern case started approx:
1897 for an A or B / Squared off fallboard,arms,modern music rack post 1911
Post 1886 for a C /C was discontinued shortly thereafter(1913) though special order till 1936)in N.Y. Never had a squared off fallboard,arms personally though they existed post 1900ish before being discontinued.
1900 for an O /Squared off fallboard,arms,modern music rack post 1907
1922 for an L /always squared off as for the fallboard and arms
1912 for an M /always squared off as for the fallboard and arms
1936 for an S /always squared off as for the fallboard and arms
Hamburg Steinways are again different to confuse the issue more so.

Confusing to say the least!
I reported the broken link as for your pic. Hopefully it can be remedied so everyone can see your beautiful piano.