I've looked at the National Archives picture for Registration 370524, and whilst the image is of a cradle, it doesn't agree with the cradle design shown in your ebay link. There are substantial differences, which you will see when I post a copy of the Kew picture. Unfortunately, although I took three separate snaps, none is really good (the Kew representation is a sepia photograph I think), and the glare from overhead lights was difficult to avoid, but believe definition is adequate for you to see the differences. As is usual, the Archive data doesn't include dimensions, but not essential in this instance I don't think.
Obviously, someone had done their homework, partly (perhaps looking in Thompson), but hadn't searched for a corresponding picture.