Find the Possession For Less upgrade

The key unlocks a chest in a house called the "Lansdowne Residence." You'll find this location from backtracking out of the Order of the Raven, over a skyhook grapple point, and through either a double doors or a skylight into a bedroom.

From the bedroom, go down two flights of stairs to the middle level. There's another bedroom and a bathroom on this floor, and the chest is in the bedroom.

In the chest is a salt recovery item and an infusion. There's also a voxophone in the room which explains why the chest (but not the key) is in the house.

I just finished 1999 yesterday and honestly what I found surprised me. It really seemed like the vigors were balanced, even in 1999 mode. As long as you are utilizing them properly, all of them are effective. If I were to focus on three, they would be Murder of Crows (for Handymen and to distract peons while taking care of heavys), Shock Jockey (allows perma-stun of 80% of enemys), and Overtow (this is critical when you get to the Hand of the Prophet, as you can literally just knock all of the enemies off the Zeppelin instead of fighting them.)

If there was one that was underpowered it was Possession. Any enemy that you posses will get demolished by other enemies almost instantaneously without dealing any measurable damage, and at a high vigor price to boot. The only way I could even use Possession was to get by some turrets without destroying them to preserve ammo (presuming you are going for the Scavenger Hunt achievement as well.)