The Accurate Timing Indicator.
The timing indicator was designed to provide the Model T enthusiast a method of accurately setting the ignition timing on his T. We are all aware that Henry forgot to put timing marks on the T engine. For those who still hand crank start their engine it is important, with the advance lever at the top, to be sure that the timing is set at 15 degrees AFTER TOP DEAD CENTER. This is as I am aware the only specification for ignition timing that Henry specified. if set this way it will alleviate the starting handle being kicked out of your hand & at worst damaging your hand, wrist or arm.
This tool can also be used to check the ignition advance relative to the Advance lever position. So if you set the advance lever at the halfway position, with the tool in the number one plug hole & the LED connected to number one timer position, turn the engine over slowly until the LED light comes on, then simply read the scale to see how far advanced your timing is at that lever setting. Now bare in mind that this is when the timer closes the circuit to start the coil operating. The actual ignition timing will be dependent on many other variables, such as RPM, coil charge rate & if you are running on Battery or Magneto as examples.