Thanks for the replies, Clive and Dave.
That's useful information, Clive. It looks like the Hazet is very top-side at best.
Dave, the spanner is from Amazon [where the others were bought from], and it is clearly marked 19 [see picture with the Draper].
I've attached some pictures, showing four different 19mm spanners [first the Hazet, then Draper, Yamoto, Kamasa] and, for further comparison's sake, two 3/4 spanners. The Hazet, at 19.35/19.36mm, seems to be a radical outlier.





