HMMM. My interest is piqued by this testing stuff. All of this is my humble opinion, of course.
It would seem very unlikely that someone would have gone to the trouble of opening the Trimline base (it's an AD-1, correct?) and fiddled with the internal wiring. The mounting cord is spade lugs at one end, and a large plug at the other, if I recall correctly. If someone would have wanted to defeat the TT dial, as you suggested, it would have been so much easier just to install the red & green leads reversed.
So, let's take a look at the connecting block. Has someone installed a modular adapter, or are you wiring the phone the original way, by fastening the leads under the screw heads?
In other words, how are you able to test the block using a modular tester?
I suggest that you get a VOM, set it to the 0-10 Vdc scale, and put the red lead on the + terminal of a D cell, and the black lead on the -terminal of the D cell. If the meter indicates + 1.5 volts, this will prove that the meter is functioning correctly.
Using the same hookup of leads, now put the meter into the 0-100 Vdc range, and test the red and green line wires at the connecting block. Put the red test lead on the GREEN (+) line wire, and the black test lead on the RED (-) line wire. You should read + 48 volts, approx. (Note, that's PLUS 48) if the house wires are poled correctly.
Remember, red is ring, which is negative, and green is tip, which is positive, in the wacky world of telephony.
Let us know the results.
