Fred Noonan's charts would be mercator's projections, scaled in nautical miles. A nautical mile is about 2000 yards, or a little over 6000 feet. Incidentally, and not coincidentally, a nautical mile is one minute of arc of latitude. (There are 60 minutes in a degree.)
The sides of the charts are scaled for degrees and minutes. A navigator learns very early not to use longitude for scale.
The only instrumentation on the Electra that I know of that would be of any relevance would be the airspeed indicator. This would be calibrated in knots, which is shorthand or nautical miles per hour. (The word knots actually derives from an older practice of counting knots in what was known as a pit log, but that is not important for this discussion.) The air speed would be measured from a pitot tube mounted on the outside of the aircraft. Noonan would only be using the airspeed to prepare his DR, or dead reckoning.
A navigator actually determines the actual speed of a vessel or aircraft from speed over ground. To calculate speed over ground, the distance between known fixes is compared with the time between the fixes. The formula is simple: Distance = Rate multiplied by Time. You can actually do this yourself. If you pass mile marker 336 in your car, and pass mile marker 337 exactly a minute later, you are going 60 miles per hour. That's because you are travelling a mile a minute. If you pass the next mile marker in half a minute, you are doing 120 miles an hour (and you are probably going to be getting a ticket.)
The navigator's calculation of speed over ground helps him or her determine headwind, or tailwind, and it's direction. Surface navigators call this "set and drift."