Rich,
You are absolutely right. I had forgotten about those photos posted by Paul. They do indeed show the bottom-hung swing hangers as you have them modeled. And your explanation of the other iron brackets being spring plank safety straps makes perfect sense.
My favorite reference about car trucks and other hardware is
The American Railroad Freight Car by John White. It is amazing how much variety there was. Mr. White includes an interesting discussion of the swing motion truck advocacy that developed for freight cars. The extra smooth riding quality of swing motion trucks sought application to cattle cars during the humane reforms era, and to tank cars to reduce risk from their dangerous contents, and to cabooses for their comfort. So there developed a kind of swing motion school of advocacy. And this strengthened the non-swing motion school, which contended that less expensive, normal trucks loosened up quickly through use, and produced the same kind of soft, sloshing ride of the swing motion trucks, but with fewer moving parts.
I am not familiar with how widespread swing motion design was with passenger car trucks. It may have been very popular with that application versus freight car trucks. I don’t have White’s book on passenger cars. EMD B-B locomotive trucks are swing motion.