There is an "eye" bolt that holds it all together - you will need 4 if using stock fenders. Don't believe they are available new anymore - I might have some.
I agree that Forged steel would be better, but a good brass casting is as good as any cast iron part for such purposes. If hard to find then it would at least work till something better is available. I finally found the information about the way the eye bolt pulls the riveted on steel fender mount saddle hard up against the forged round bar support brace. Original eye bolts seem rather oblong but I wonder if they stretched out over time. Some of that stuff is likely on Flea-Pay. The forged support bars are anyway.