Thanks to those that pointed out that this piano, if accurately dated to 1890, most likely has 85 keys. I had it in my mind that 1890 was the first year for 88 keys on a B, but 1891 is what the research shows.
to close borderlines for 85/88 key questions.. ;-)
..1892 is what the research shows. See below.
Also in the year 1890 there were several "88 keys" Steinway pianos.. to be honest: most of them. The grand models A and B seem to have been the "last Mohicans" with 85 keys - until 1892.
The concert grands yet had 88 keys in 1863 and afterwards never again had less than 88 keys.
The real model C (introduced 1886 as a smaller derivate of the new D from 1884) also had 88 keys.
And, yes, in that years some so called "square grands" also had the 88 keys. From 1885 to the end of square production in 1889 there existed new squares only with 88 keys. the biggest and probably best and most powerful squares ever built!
Some bigger squares had 88 keys yet four years after start of New York production in 1853. Eldest big square pianos date back to April 1857 with 88 keys.
The other way around: the last entry for Steinway pianos with 7 octaves (85 keys) is written for a B model which went into store 1892/11/18, serial # 75473 (highest serial no., but finished a litte bit earlier was #75627).
So there was a time of some months overlap in 1891/92 in building A and B grands either with 85 keys or with 88 keys.
I could not find any mentioning of 7-octave pianos after this date, also for the uprights.
So, all Steinway pianos with higher serial numbers than 75627 should have always 88 keys if I read it right.