So I just wanted to close the book on this thread. I installed my "new" Beru loop GPs which were made in W. Germany (so a few years old, ha ha). They are definitely better made than the current Bosch loop plugs from India. They have insulating material (maybe glass?) to keep soot from bridging portions of the plug, which Diesel911 mentioned above. Unlike something I said in my last post, these old Beru plugs are black like the Bosch's. I have no idea if newly manufacturered Beru plugs are identical to what I got on Ebay, but I am pretty happy with them.

I also decided to clean all of the bendy connectors between the plugs with steel wool. They had 45 years of grime on them, so it was probably overdue. I don't know why it didn't occur to me to do this last time. Anyway, it was an easy job which greatly sped up the glow process. I now get red glowing on my salt shaker indicator in about 8 seconds, which is 1/2 as long as before.

I used antiseize when installing the plugs, as I found an old MB document saying you should use graphited oil. 35 ft-lbs is the correct torque for these older plugs. Thanks again everyone for your help. It is nice to have the car running again. Now, I can work more on resolving why my car runs so warm.

__________________
1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).