[Elecraft] Jack on bottom of K3?

[David F. Reed]

I note a jack on the bottom of my K3 (between the VFO B and the 
clarifier tuning knobs) that looks like an Ethernet or wide phone jack 
to my fuzzy vision...

And, I seem unable to find reference to in in my manual.

Any ideas what it is out there?

thanks and 73 de Dave, W5SV