I had a difficult time understanding where this image was taken from and it took me awhile to realize that is because nearly all of the buildings in foreground are gone. A photo in the Minnesota Reflections database matches the postcard almost exactly but with a bit of a wider view. This suggests the image is from around 1905, showing the area that would soon be home to the St. Louis County Courthouse and other buildings of the Civic Center and before the widening of Mesaba Avenue. The only building in the foreground that seems to still be standing is the house in the lower right corner, circled in blue in the Google Earth screenshot from approximately the same perspective. 

The postcard photographer seems to have taken the photo from the stone wall still standing on the north side of Third Street between Seventh Avenue West and Mesaba. The bottom image, also from around 1905, shows the Incline Railway on Seventh Avenue. The likely location of the photographer is marked with a green X and the large house on the left side of the postcard is circled in red.
 

I’m also impressed with your attention to detail there. The two images are so similar that I thought the photo that was used for the postcard might have been a different shot taken just a few moments apart from the Minnesota Reflections photo. But after you pointed out the McKay Hotel, I realized that a whole game of find the differences can be played with the two images, all of which involve buildings or objects (like a prominent water tower) that appear in the photo but not the postcard (which seems to be a photo that has been colored in). 

So the postcard is likely from before 1901 when the McKay Hotel was built but after 1894, as the old Federal Building seems complete in the postcard.