Back to Robb for his take, and sure enough, he had a possible answer:
I think that might stand for “ South River Brick Company.” I’m not sure, but it may. There was once a company in South River N J that made enameled bricks. It’s near the town that I come from, Sayreville n j which is known for Brick Manufacturing as well. Bricks from Sayreville are embossed S&F B C. ( Sayre and Fisher Brick Company).
Also in this photo are two square-shaped impressions in the wall, behind which is the chimney. It will be interesting to see what's behind those two squares.