It has finally been worked out (thank you all for suggestions).
These are slugs. Or maybe lugs. This is the other one I have with me.
We are looking at if from the bottom - from the other way it can be lifted out of the fire box when red hot, with a device like a hot-plate lifter.
So, even when heated and expanded, it was just under five inches.
It was made for, wait for it, a five and a half inch slug iron - which is like a coal or box iron, but without the chimney.
And those three little feet were on the bottom, so it was radiant heat that was given out evenly. And you could even put a couple in, one above another. But one was good enough, as you mainly wanted the heat at the bottom. But more lugs = more weight = even better ironing.
There are three Slug irons shown at the bottom of the page HERE
And the oval hole in the centre meant that it heated up quicker in the centre, and you weren't left with a colder spot in the middle of the s/lug.
This one has had a hard life, and looks like it has been overheated a few times - maybe it even got left in the fire too long.
So - now the hunt is on for more lugs.
You can see the other one in the set HERE