I believe that I have a manufacturers bridge plate from that bridge. At least the date lines up with this bridge. I dug it up in my backyard when I was living in Ontario, California 20+ years ago. I don't know how many bridges the Midland Bridge Co. built in 1909 so perhaps I'm wrong and that it is from a California bridge
Most likely from a bridge built by them in that vicinity. Not impossible that a plaque could meander that far from home... but highly unlikely. I was looking at their page on BH and they were building out in Arizona, so I'm sure they probably erected a few in Cali as well.