What think ye of this tester? Who is STAF Ford?
Snyder's Old Car Parts sells this for $135 new. They have an instruction sheet that can be downloaded. It will test for amperage draw on the coil. It won't test capacitance or check for double sparking.
It's generally called a buzz box Tommy and there are many of them around both new and old. The built-in sparkgap will show if the coil can produce a spark and the meter is used to monitor the coils' current draw when adjusting the lower point contact. This kind of tester is not capable of showing double sparking which is then removed by adjusting the cushion contact. Money spent on any buzz box is money wasted. Many people swear by them but that's only because they don't understand proper operation and adjustment of the contacts.
About all it will do is give you a coil capable of running on battery. It will NOT ensure the coil will run well on magneto.