Neil, it 'looks' right. My adapter seems like a cheap version of the same. It has a depth of 26.3mm producing an overall backfocus, when fitted of 55mm.
If you have a look in the Terrestrial Images section here, and check a thread titled "Taking a risk" you'll see a recent image taken in a howling gale with a cheap 1000mm lens (hence imperfect focus) so you can see it does work.
Here are images of my adapter.
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