What is this gizzmo the tach wire runs through?

[jscott]

My car has a factory tach but was not born with it.  The prior owner set this up, it works but I just want to know if it is a factory thing or something that was added.  I is white, feels like ceramic to me.  Wire runs from the positive side of the coil to it (red wire) and then the wire from the tack goes to the top of it (green wire but looks mostly black due to the shrink tube).

 

[Don C]

It's a ballast resistor. Apparently during the conversion they eliminated or bypassed the factory resistor wire.