Alan:
I had one like that about 40 years ago. The smaller size allowed Spridgets with folding tops (like my old '67) to drop the top with plenty of clearance for the roll bar.
I will say that I did not really care for the way it looked and gave it away, replacing with a full-width roll bar that was floor-mounted, as Bill explained.
But one point I should make: Modern Spridget racers often mount the "halo bar" on the shelf (like the one you have) rather than all the way to the floor. The shelf behind Spridget seats has a reasonable amount of strength. I see SCCA and vintage racers with this setup all the time. But the bars are much wider, and tie into the area just in front of the rear inner fender. And these race cars generally have full roll cages, so it is a somewhat different arrangment than a single hoop.