Technical SW Gauge calibration help

I installed a Stewart Warner voltage gauge in my 56 T-Bird. The problem is it reads low. I have checked it with 3 different test gauges pulling power and ground from the back of the low reading gauge. The 3 test gauges all read the same 12.6 volts, while the SW gauge reads just under 12 volts. On the back of the gauge near the bottom and centered is a nut, I was wondering if it is used to calibrate the gauge.

 

This may help, the guy got a defective gauge and he's checking the one that replaced it.

 

Thanks, but that is basically how I tested mine. Bought it on eBay over a year ago so no returning it. Hoping to be able to fix it or re calibrate it as it just reads low.

 

Contact S-W , I was able send to,
4 different gauges I Believe, S-W checked & calibrated and returned, Now I cannot remember if they paid /sent a return back to check (S-W) shipping label or not. it was about five years ago.
This way # I used
1 (800) 676-1837

just asking have you tried to check
12, 13& so to 16 V to see if reading was .6 volts low @ each volts scale ?

 

You might want to post your SW question over here, where the collectors hang out:
https://www.jalopyjournal.com/forum/threads/early-stewart-warner-gauge-thread.1019310/page-17