It's my party wall and I'll cry if I want to...

So, when I tore out the ceiling in my back bedroom, I wasn't necessarily surprised that I might find a small hole or two in the 93 year old exterior walls. But I have to admit that I was a bit surprised to find an entire slot behind the plaster in one of the side wall. The condition exists where the party wall, which is shared with the 1908 house to the right, ends, and is met with a newer exterior wall built as part of the construction of my 1914 house. The walls are built like this to provide for expansion between the two walls, a practice that is common today. What is surprising, however, is the lack of any sort of sealant, beyond the plaster on pulled-wire mesh on the room side of the house. If sealant was used, there is no indication of this that remains. For now, I've used Great Stuff on the interior, and plan to use sealant on backer rod on the exterior. Hopefully this will be the last time I'll have occasion to say, "What light through yonder wall breaks?"