Since writing my original work on clams, I've found that there are some derasas that have scutes, especially when small. I've encountered many of these diving on the Great Barrier Reef a few years ago, and then found that they were being farmed in a couple of places, too. The pic is of one at a farm in Tonga.
It's very interesting that every one of them with scutes also has small, round, light-colored to clear "windows" on the mantle. It you look, you'll see them.
They ARE NOT hybrids. So, either they're a variant of derasa, or a separate species. I am working with someone right now to have some genetic testing done and hope to have an answer in the next year or so. It's not fast (or cheap)...