I hope to post a few pictures here of real roll bars used "back in the day" when things were simpler and there may have been very few standards for roll bars. This first one is "beyond its time" in the sense that it is not a very sturdy design, but it might be "better than nothing". There are multiple copies of this one around, so it may have been a commercial aftermarket part, off the shelf and bolt-in ease of installation.

There are a few problems with this one. First, I don't know if the folding top would actually reach over top of the roll bar. Second, the top frame obviously does not quite fit in between the legs of the main hoop. The top might stow in that position behind the seats if you left out the side curtain stowage bag. Third, there is no side to side angle brace in this weldment, so side impact strength is limited. But the more serious concern is that this roll bar is not attached to the chassis frame. It is bolted to the body sheet metal on the battery shelf aft of the door opening and in front of the inner wheel arch. It is likewise bolted to the sheet metal on the body tonneau panel (not extending through into the boot). In event of a roll over this would likely cause a lot of sheet metal damage, bus as said it might be better than nothing for prospects of saving your life.

In today's light, I view this one as "fuzzy dice", or "an appearance accessory that does not materially affect the performance of the vehicle". For most modern competition sanctioning groups this would only be allowed in some Stock class where a roll bar is not required at all. It would not pass inspection as useful for actual rollover protection.