In his letter to Anna, Oskar describes the atmosphere and culture of Niels Bohr's Institute, the "foster home" of the fledgling quantum mechanics.
The Institute seems ordinary enough from the outside; there is nothing to distinguish it from its neighbors, nothing to warn a passerby of the madness that reigns within these walls. Often I think we should follow the custom of Copenhagen’s shopkeepers and hang up a metal sign above the front door; but what symbol would we employ? The pet store displays the silhouette of a parrot, grapes announce the liquor store, a pastry dangles above the bakery, and a watering can and kettle are suspended above the metalworker’s shop: each simple image announces its occupant’s vocation and business with clarity. Part of the trouble with quantum mechanics is that we don’t have such a symbol yet. The world that exists this side of the threshold is surreal, but it has a strange logic all its own—unexpected, but self-consistent.
There's a charming little video on YouTube (courtesy of British Pathe) that shows the signs Oskar is talking about. Click on the picture below to watch it.