Alright, here’s my best guess, and I think it’s a good one.
This building sat facing Acorn St, in the middle of the intersection of what is now Saline and Alexis St in the Run section of Greenfield, and was then the intersection of Alexis and the unopened Right of Way of Forward Ave (which did in fact continue all the way out to Squirrel Hill but has not done so since the Parkway was constructed).
There is no other explanation I can come up with for why they are putting a sewer main under a building!
Here is a map of Greenfield Sewers, showing that line: http://www.pgh2o.com/maps/Greenfiel...
And here is a map of the neighborhood in 1904 showing buildings in the right of way, I think there may even be a manhole drawn in front of the building in question, meaning this construction was replacing an existing line:
Great detective work Matt! Your evidence is pretty compelling and I have to agree with you on this tag