Finally figured it out. A lot of wasted time looking for some kind of mechanical means to bring the declination arc to zero when there is NO mechanism.
Process is to just remove the glass cover and rotate the OUTER CIRCLE back to zero or to the correct declination on the declination arc, or as the Deightzgen illustration calls it, the variation plate.
While the text that accompanies the illustration extoles the benifits of this variation plate it is actually the graduated outer ring that moves and there is no mention anywhere on how to move anything.
Thanks to all for your helpful ideas.
PS. I tried this procedure early on but the graduated outer circle did not rotate freely and I was reluctant to use more force to get it to rotate. As it turns out that added force is what is required.
OK, just finished smacking myself just like Joe Ferg predicted. All that is required is to unscrew the glass compass cover and manually rotate the outer ring to the desired reading on the declination arc. I tried this early on but it not move easily and I was reluctant to force the issue. Quite a bit of force is actually what is required. I finally found a 1931 Deightzgen catalogue that had an illustration that describe my transit as having a "variation plate" that allows you to adjust your instrument to compensate for declination but has no information on how this "variation plate" is utilized or what the actual process is.
In my view, this plate has nothing to do with moving the graduated outer circle and is not in any way an "exclusive feature" of the Deigtzgen transits and describing it thusly sort of gets you focusing on the compass face and not the outer circle which is where the action is.
Thanks to all for your helpful ideas.
You said you have already found two monuments. You might consider sighting between them to see how the bearing compares with what your survey drawing says it should be.
That page from the K & E Paragon manual posted by Norman OK doesn't translate to a Dietzgen transit for declination. I also have a 6100 and after several minutes of looking for the declination capstan screw, took a close look at the parts & assembly diagrams on pages 16-23 (I downloaded the manual), saw that the vernier windows on the 6100 are a little wider such that the edge of the window on the "South" side is where the capstan adjustment screw is shown to be in the manual.
The adjustment mechanism seems to be well-hidden on the 6100. I think that the bent spikey looking thing is just what Bob Freeman guessed it was, protection for the level vial. There can't be anything underneath as that's the housing for the horizontal circle and is stationary relative to the base rather than to the alidade.
Since there is a declination indicator in the compass box (mine is set to zero) and since the one on dinasnore's is set to 13 degrees and 50-some minutes West, there has to be an adjustment mechanism on it somewhere.
I used one of these things for my first couple of years surveying (early 1980s). But either because we were in central MI where the declination was between 0 and 5 degrees, or because we were typically working with highway plans and from established control (MDOT), declination was something we never dealt with.
I now have a new mission (on a longish list of missions).