Ron,
Here is an illustration which may be helpful.
The first part shows where the serial number is on the trigger (hard to see the number itself in this sample, sorry).
The second points out the sear bar, if I understand you correctly the number is stamped on the bar and is hidden by the sideplate.
The third part shows where the serial number will be found underneath the front toggle.
By the bottom edge of the takedown and sideplate, I mean numbered as you see those parts here:
The D stamp you have noted is what is known as a "worker's mark". They were stamped during production for reasons which are unclear, probably internal inspection of some sort; they are not useful in helping identify your Lugers.
--Dwight