Yes, two sets of holes in the case. Thanks! That clears up that mystery.
Let's forget about the pin (although I will leave it there).
The rigid arm set-up uses an elbow that looks very much like the elbow on a front-mount machine. That makes sense because this design is intermediate between the travelling horn design and the fully developed rear mount set-up. On those machines, the horn looks heavily cantilevered on this point, so the idea that this attachment held some sort of a horn crane makes sense to me. Wouldn't this mean Victor made horns with the means to hook them on? I haven't seen a Victor horn with a hook, but then there's lots of things I've never seen.