PC3448P EPS-470 pinout

It's just 3 pins, and I have successfully identified the +/- ones, what is the third one for?  I'd like to power the switch from -48V DC telecom battery backup instead of the original EPS-470, and it works but I can't get redundant power (both AC and DC at the same time) to work properly, I can hear a relay inside click very often (which may not be good for the hardware, so I don't leave it running in this condition for a long time).  Has anyone reverse-engineered it alrady?

Middle pin is positive, the pin closer to the AC input is negative, and the third pin is not earth ground for sure.  Left open, it's about 2.5V above the -48V DC rail.  So it may be some kind of logic input or output (or even an analog signal for some kind of load sharing?) between the switch internal PSU and the EPS-470.

I know what I ask is not officially supported, I'm prepared to void the (already expired) warranty, I'm an electronics engineer, aware of electrical safety issues etc.