It could have been armored by an outside company which would not be reflected in the mb data. If you are certain it was armored and that Russians came to take the armored parts , to swap with regular panels , maybe you can contact the Russians
I know it can be done, there's a place near here that told me they had a small fleet of armoured W126 in years ago to have the arbour removed. They did the job and then apparently all the cars needed new springs as went 4x4 style high.
Dunno about the glass question though, have wondered before. There was an armoured one over here for sale for about £20k I think it was but the windscreen was crazed so bad it wouldn't pass the roadworthy test. I did wonder what on earth you could do with that. You could have a new screen custom made apparently but imagine the huge cost. So cheapest would be retrofit of a stock screen, but what about the channels it would sit in etc, different thickness must have meant adaptation I thought.
420's were used as armoured versions plenty although I'd guess not as much as the bigger engines. I saw a 420se armoured version once with cloth seats, and other very cool options like a microphone in the glove box to address people outside the car.
OP has your car got any other interesting features? Evidence of a PA, odd switches, two internal rear view mirrors or anything?
I do wonder what they meant by special upholstery too. I don't think the colour is a special order. (It's awesome though)
So I had another long chat with Pops reminiscing and trying to pick up details of the story of the carThe Russians! Of course they would exchange stock W126 parts for armored ones, while trying to sell nuclear subs to the Colombian drug cartels in the early 90s.
But seriously, can you really return an armored car to original just by swapping parts? And by doing that in a non-factory environment, leave no obvious marks? I am thinking mostly about the laminated front and rear glass, floor and roof armor plating, additional fuel tank protection and the list could go on depending on the armor protection spec.