Accompanied by GORDON COOPER’S signed provenance letter on this personal stationery which reads: “This Douglas SKY RULE was given to me by Mr. Don Douglas of Douglas Aircraft in the 1950’s while I was an Air Force Officer. I used it during many Air Force and NASA jet aircraft flights. It was a convenient tool to compute air speed, mach number, pressure altitude, and other important flight parameters. I found it to be a very advanced and useful instrument for the period it was used.”
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