The Maine Turnpike opened in 1947 as a new highway between Kittery and Portland and later was extended to Augusta. These photos show what the point of the extension looked like in 1956 and what it looks like today. 

The Maine Turnpike tweeted this picture showing the highway from the Congress Street overpass in 1956. This is where you'd see Unum to the left today.

We're not sure exactly where the extension began, but the original Turnpike was 45 miles long which would put it right about in this area, so this photo may have been taken to show off the newly complete construction.

Here's what that same section of highway looks like today.

There's a lot of growth over the years on the left, a reconstructed overpass which now has fencing and Unum can now be seen on the right, and lighting has been added to aid drivers coming onto the Turnpike from a new exit was added at this location much later.

And here's a photo mash up of both images.

Maine Turnpike/Google Street View

If you want to learn more about the Maine Turnpike, they have an interesting history of Maine's first highway system on their website.