What we see now is, presumably, a rebuild using as much of the original stonework as possible, but how did they know what it looked like down to the finest details. Did the rebuild use original drawings (probably unlikely) or was there a real effort in the time leading up to the war to photograph everything from every angle in order to prepare for the worst?
I worked in the Preysingpalais in Munich for several years (Residenzstrasse 27, right behind the Feldherrnhalle). The stairwell is mentioned in several guidebooks as an example of Baroque artistry, we got tons of tourists viewing it and admiring it in whispers while its acousitcs are remarkable and it's well worth speaking up there. Anyway, it was completely rebuilt in 1963 following the original plans which had been stored away with other documents from the Staatsbibliothek. The real estate was bought up by a Swiss investor for a song and a half under the proviso that he had to rebuild the Palais as true to the original as possible, especially the facade and doorways had to remain unchanged.
Take a look at it next time you're wandering towards the Theatinerkirche; you really would think that it had survived the bombings while in reality it was 99% destroyed.
P.S. This is the spot in Residenzstrasse where cyclists and pedestrians tend to collide.