I wonder when she actually got it. Eddy wouldn't have had it with him when he proposed, I don't think. Then, after he left Luton Hoo, they didn't see each other for a few awhile--a week, maybe? Perhaps he gave it to her then. They had already been photographed once while at Luton Hoo. I think they only had 1 or 2 more photos before he died.
Plus, as I mentioned elsewhere, it's almost impossible to get a good look at the ring George gave May and she wore that for decades through 1000s of photos! Perhaps the 'Wurttemberg hands' that Louise commented on made May reluctant to have her hands photographed? She's almost always resting them in her lap, holding a fan or some item, or wearing gloves.