South Bend made open side ones like that. You can see it in this 1926 catalog.
scan (11)
The one in the catalog does NOT match up with the one you are looking at.
The following picture shows the measurements you need.
The red one is from the top of the V to the centerline - this will tell you if the steady will center on your spindle.
The green one will let you know if it fits on your bed.
Steve