As I wrote, I was not sure where to even go looking. Found your post, and it's a quiet weekend. Went searching. With my eyes glazing over, and about to quit, I ran into this:

https://www.thalesdsi.com/our-services/communications/hf_communications/

So, what the heck is an NVIS antenna? Off on a new search. Got nothing definitive. However, if you read descriptions of the antenna, and see the NVIS antennae installed on other systems, you see a strong similarity between those and what's on the cutter. Finally, I looked at all the other antennae on those cutters, and that inverted U is the only thing that looks to fit the bill. I have absolutely zero proof, but at this point, I strongly suspect it is the NVIS HF antenna. I am done searching. Good luck to you.