definitely 1 1/8"?
Have you verified it takes a 1 1/8" ahead stem by actually fitting it inside one? I have a couple of quill adaptors for 1" threaded steerers. One fits a 1" stem and the other 1 1/8" - and yours looks like my 1" version in that pic. 1" quill adaptors were generally made to take 1" ahead stems but were supplied with a shim for use with 1 1/8" stems, especially older ones.
£5 posted sounds right though, I paid about that for both mine.