The annex was numbered after it had been divided. The central space already had an instrument bolted to the floor, and nobody wanted to disconnect it to discover whether it had a room number underneath. The surveyor therefore numbered the two usable rooms and described the space between them separately.
The plan was later copied without its legend. That copy is responsible for several confident claims that there were only two rooms. Confidence does not count as a measurement.
The complete plan includes doors and electrical points. Match both to the receiving description, rather than choosing the room with the most ordinary number.
Notebook question: Which room has both opposing doorways and the six-socket point?
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Retained correspondence · Equipment trail · Accession copy, not a new event