Ok, I've found the gauge in my 707 manual. And according to its scale and position it has to be the rudder hydraulic pressure gauge. As you might know, the rudder is the only control surface in the 707 which moves by hydraulic pressure. The elevator and ailerons are moved by cables.

But there is also the "start air pressure indicator" in my manual! It has the same scaling (0-4), and its in the same place. BUT, it got two needles; L and R.

EDIT:
As for the EPR-bugs I've found some pictures. Some had them, some didnt:
http://www.airliners.net/photo/Sabena/Boeing-707-329/1555432/L/
http://www.airliners.net/photo/Untitled-(Omega-Air)/Boeing-707-3M1C/0736382/L/
http://www.airliners.net/photo/Air-France/Boeing-707-328B/0397633/L/

Again, they had them, if they were either retrofitted or ordered with them.