I was thinking the same, that they did a good job in integrating the expansion to the rest of the building. There could have been some detailing added to the top to emulate the original building, but that's nitpicking on my part.
Just looked around the net to see if there are other pics out there, and found this older one, from a postcard, which appears to show that the staircase didn't extend to that area originally.
Source
Not sure what the "Dartmouth Federal Building" referred to - perhaps it had some customs function?
I have a vague memory that the MT&T buildings went up sometime in the 1970s, can't remember for certain.
Not sure about the war memorial. I walked by there recently, but didn't take notice of it.
Am curious about the future of post mail - on one hand people don't tend to write letters anymore, and many people pay bill online, but on the other hand online shopping is kicking the tar out of brick and mortar stores, so it could go either way - probably mostly depending on how they compete with the courier companies.