Hi Silverfan,
For what it is worth, here's a few snippets of information regarding A.D.Hauptmann & Co:
In 1894 the firm was in the hands of Franz Hauptmann and Anton Richter and the same partnership existed in 1897.
Around early 1903, Anton Richter died and the firm was continued by Franz Hauptmann alone.
Their premises were located at Kohlmarkt 16, Wien in 1903. They were perhaps out of business, or just relocated, by 1912, as Kohlmarkt 16 was remodelled into the bookshop 'Manz' (the facade was designed by Adolf Loos in that year).
Kohlmarkt 16 was earlier the premises of the bookseller, Ludwig Auer in 1820:
Ludwig Auer - Wien - 1820
It is now, I believe, occupied by Louis Vuitton.
Trev.