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Back in the 1990s, when I was still on active duty (USAF, not USN), I
looked up mil spec for the Navy Mark III sextant. The spec appeared to
be a clone of a C. Plath. There was a name (maybe a trademark?) for the
metal of their frame, and the mil spec required that same metal. I think
it also called for their ergonomic handle.

To me it looked like a case of "specmanship." Such manipulation of the
military procurement system was an occasional tactic. You couldn't say
"I want this model from that manufacturer." The correct procedure was to
state the what properties the device had to possess. Then the supply
people would search for the product that met your requirements at lowest
cost. By carefully crafting the parameters of your request, you could
guarantee the only match would be device you wanted.

The mil spec for the Mark III included the test procedure. The sextant
was clamped to a precision rotary table, with the index mirror at the
center. A collimator, also attached to the table, was aimed into the
horizon glass to simulate the horizon. A fixed collimator aimed at the
index mirror simulated a star. By rotating the table you could control
the altitude of the "star." Tests were made with and without shades. I
forget the maximum permissible error.

   

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