Hello
It is most likely out-sourced. I don't know who would have manufactured it. Before my arrival in late '86, they made bar / stem combinations that were installed on some '84's and '85's. I never welded any of those myself and I don't recall seeing any in the shop when I started.
We did make stems but mostly for our own use and we always used 2 bar clamps and not a single like in your photo.
At some point, Chris just decided to go with Salsa stems when we were supplying complete bikes. We worked with Seattle Bike Supply for a while to provide component groups to dealers. I'm not sure if that stem was included or a Salsa. That was not my area. There were some saddles made that had FAT CITY printed on the back of them. During the Somerville era, the name of the company was Fat City Cycles and Fat Chance was the model name (along with Wicked, Yo Eddy, Team Comp, etc)
The stem looks like it has decent welds anyway
Regards
Scott