This is an example of the letterhead that would be used by the Sanitarium's administrators for any outside correspondence. The PDF includes an embedded typewriter font (the excellent "Chunk Type" from John Bruce) so the form can be filled out before printing. You can download the full PDF via Google Documents over here.
The address of the Sanitarium on East Derby Street is based on Gahan Wilson's interpretation of Lovecraft's original hand drawn map of Arkham. The street number (345) is taken from Butler Hospital in Providence (345 Blackstone Boulevard). Butler was the place where both of Lovecraft's parents were treated and it was a major influence, if not the direct inspiration, for Arkham Sanitarium.
The telephone number draws on John Singleton's excellent research into conjecturally accurate exchanges for the classic era and later. "1893" is the year Lovecraft's father began his stay at Butler Hospital.