You do in fact have a floor tom leg mount, and not a proper knobby for the cymbal arm. But what the others have mentioned is application too. The knobby cymbal mount has essentially no space between the plate and the clamping part. The floor tom leg has the distance between the two in order that the leg will clear the hoop of the drum. The leg mount is not a good choice to use as a cymbal mount, because the distance between the clamping part and the plate will tend to induce more torque into the mount and it will fail earlier than the proper one.
That is indeed true. But after a while they discontinued using the short mount and used leg mounts instead. And when the cast leg mounts arrived in the early 70's, they used those for the cymbal mounts too. This bass has the later t's and claws, it's probably a '68, maybe later. That style mount should fit, but if it doesn't, then the mount itself is too early a model for the holes in the shell.
If the hardware shown with the mount is original to that mount, those square nuts indicate a very early mount from the b&b era. In that case, the holes in the mount won't line up with the holes in the later shell.