Thanks for the advice.
I think the caps are attached or, if not, the cap on the rear burner is stuck on tight.
Our home gas cooktop has similar base and cap, though the caps are cast iron. The propane comes up in the middle of the base, like a Bunsen burner (ask me how I know!)
I do like the looks of the cooktop; the continuous cast iron grate is great (pun intended). The stove itself is an improvement over the one that came installed in our Leisure Travel Van. It had light separate grates over each burner. They fit into the top in grommets that broke and were expensive to replace. And, the stove was set into the counter with a black rubber (or some material) seal around the edges. If you got a pan close to the seal, it melted! One con I have found with the Oliver stove is that the writing on the control knobs comes off if you wash them with any kind of cleaner. It sounds like Oliver has the best choice available.
Once I hear from Oliver, we will probably pull the cook top out and inspect to see if any repair is needed.