As I understand it, and IIRC, the mechanical IVR takes the 12-14 V input and regulates it by going ON-OFF-ON-OFF-ON-OFF-ON-OFF ad infinitum. The average voltage that comes out is about 5V.
Similar to the point type mechanical regulator on the firewall.
Gotchya. What I've read is for test and calibration, companies use 6v. That's why I'm going with it. Makes sense. To me. LOL I'll find out if mine ever bites the dust. So far, it's rockin right along.