Wow, lots of research being done! It is a tough one, so many look-a-likes.
Amazing Janet how yours survived the winter in your greenhouse!
The flowerbud on yours does look like mine, just more elongated. What a huge flower in relation to the cactus body! Beautiful and nice capture too, white flowers are difficult.
The photo from this link (Echinopsis obrepanda)
really looks like a perfect match to me!
Janet and Leslieray!