Here's diagram of the Apollo white room. The white room, located on the end of the Launch Umbilical Tower (LUT) swing arm #9, was an environmental chamber through which technicians, support personnel, and crew members accessed the Command Module.

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Adapted from p. 1-2 (p. 8 in the PDF) of the Apollo/Saturn 5 - Launch Complex 39, Mobile Launcher Service Arms, Operations and Maintenance Technical Manual
Extraction and clean-up by heroicrelics.org.