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The sign accompanying the Mercury mockup. It reads

Mercury Engineering Development Fixture

When McDonnell Douglas engineers began the design of Mercury, no such craft had ever been built or flown before, except in the minds of science-fiction writers. As their ideas took shape, model builders converted them to hardware, fitting each tiny system, tube, electronic box, switch and control cable into its proper position to insure that they would fit in the small space available. This full scale mock-up, a vital engineering tool, was the result.

Mercury astronauts frequently came to St. Louis to sit in this mock-up, planning their missions, helping to position the switches and controls, and being fitted to the form-fitting couches necessary to withstand the gravity forces of lift-off and reentry.

In addition to this mock-up, individual systems were built in mock-up form for additional testing and evaluation, a practice which has long been standard in aeronautical engineering.

Of course, McDonnell Aircraft Corporation didn't merge with Douglas Aircraft Corporation to form McDonnell Douglas until 1967, so it really would have been "McDonnell engineers" who were designing the Mercury spacecraft at the time.