The Harper Piano Co of Holloway Road were active in the first half of the twentieth century, manufacturing a range of instruments including gramophones and 'synchronising pianos for cinema theatres'. The company later changed its name to the Harper Electric Piano Co, not because it was making an Edwardian electronic keyboard but because it made instruments which could only be played after putting a penny in the slot. In the year to November 1905 they sold 55 of these, mainly to cafes and restaurants.
Although the company is long gone from Holloway Road, the sign still stands out. It appears to have been overpainted at least once.
Image (left): 1922 British Industries Fair advert, via Grace's Guide wiki.