I have been looking for a similar bracket for my 74 Mustang smog pump. It's a rare piece as it's CA only. I spotted the Boss brackets and their prices. I stay away from them.
Last week at Carlisle, I found a similar bracket next to some Boss brackets. I thought they looked very similar. The one I bought said 351 C. For $10, I took a chance or maybe I could modify it.
Looking at your pictures, yours is reversed from mine. Possibly reversed from a Boss bracket as well. That said, I would think it goes on the other side of the engine (drivers side). That might make it a 6 cylinder bracket, is my guess.