Enigma

by Nika

I like to think of myself as an intelligent and observant person.  So, imagine my recent surprise upon discovering an enigma in my old house.  I have spent so much time in 815 17th Avenue that I should know every crack and cranny intimately.  Nevertheless, I have found this large keyhole-shaped opening.  What could it be?

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As you can see, it is located just to the left of the front door on the interior.

It can’t actually be a keyhole.  It’s the wrong size and the in the wrong position.  Anyone know what this can be?