As GHST1 noted, the writing on top of the mystery building is not clearly legible in the picture that he linked to, but if you zoom in on the writing on the top of the building to the left, it says F.A. Patrick & Co. This building also looks to be about twice the size of the building in Paul's comments, but as the front half matches in both pictures, it may have been added on to by the time of the later picture. 

Is the claim that F.A. Patrick & Co. also owned the mystery building to the right, or is the mystery building still a mystery?

Thanks for the clarifacation Tony. I mistakenly thought (and perhaps GHST1 as well) that the picture of the F.A. Patrick & Co. building that Paul posted in his first comment was supposed to be the mystery building but I understand now that you were using it as a reference point to identify its neighbor (Leithead Drug, later Northern Drug).

According to a listing in the Bygones section of the DNT, the city completed demolition of the Northern Drug Co. on Dec. 29, 1977, as part of the Gateway Urban Renewal project. 

As GHST1 and others noted, most of the other buildings that were a part of Duluth’s urban renewal had already been demolished and replaced by this time and that would make it one of the last buildings to be taken down as a result of the Gateway Project. But apparently not the last. 



Here’s a photo from 1975 (note that the completed Ordean Building is visible in the photo) where both the Drug Co. building and its smaller neighboring buildings are still standing.



And then here’s a picture thought to be from 1982. If you look directly under the nose of the fighter jet, the smaller buildings that were next to F.A. Patrick & Co. and the Drug Company are still occupying a corner of the DECC parking lot. I couldn’t find any information about when they were finally demolished, but it seems like these two small buildings must have been the last casualties of the Gateway Project.

As a technical question, there used to be button for attaching a single image with a comment. That is now gone, but other people are still posting images with their comments. Is there a new way of doing this? I like to include cropped sections of the historical photos with my comments.

Your theory is most likely correct. According to the 1958 directory posted by GHIST1, there was indeed a Minnesota Power and Light sub station at 410 W. Commerce St., which would have put it right in that row of last remaining buildings.