The three pin header was just to allow use of the second set of contacts. My feeling was that a vast majority of the users would not need or use them. The resistor is indeed sized for 18 volts, for a lower voltage, the resistor would also be changed in value. I suppose I could leave that one unsoldered and include a couple of choices in the package for various input voltages.
Stan, I thought about an "enhanced" model, but I've found this to be pretty useful in our own setting, so I figured I'd float this one first. 
I just realized I didn't address the last part of your comment Stan. I'm using the HK19F-DC12-SH2C with the 720 ohm coil.
