Is Arkham a Real Place?

Yes. Arkham is a genuine town on the Massachusetts North Shore, and this office sits inside it. It carries the SUPPRESSED stamp, which is how the Archive records a place that exists but has been scrubbed from public maps and records. That is a different thing from a place that never existed, and the difference is the whole of this answer.

What the four stamps mean

Every locality on file carries one of four stamps, and Arkham's neighbours are easier to understand once they are set side by side.

• VERIFIED — an ordinary place that can be found on any map. Providence and Red Hook are filed this way.
• SUPPRESSED — genuine, but removed from public maps and records. Arkham, Kingsport, Innsmouth and Dunwich are filed this way.
• UNCHARTED — never surveyed, or surfacing only once in a while. R'lyeh is the plainest case.
• UNLOCATABLE — a place the witness could not find a second time, or declined to name.

How Arkham is known to be there

The evidence is the volume of testimony. Six chronicles on file are set in Arkham itself, and its boarding houses, burying ground and medical school have records of their own: the Witch House, the Derby House, the Old Burying Ground and the Miskatonic University Medical School. Many more chronicles pass through it. The University stands there and so does this office, which is the most direct evidence this office can offer.

A researcher who wants a point on a modern map is usually directed to Salem, in Essex County, and the Field Survey pins Arkham there as a reconciled position. That mark records the reconciliation of a suppressed town with the surviving one on the same ground. It does not claim the two are the same town.

What a visit finds

An ordinary visit to Salem finds a town with its own long history and no University gate, no Miskatonic River and no Witch House of the kind the chronicles describe. This is what a SUPPRESSED stamp predicts, and the Archive does not treat it as evidence against.