Thanks folks ... interesting.
I've seen similar photos for sale in antiques fairs.
Mine measures 6" across, and it is set into a circular piece of metal. There's a metal wire device attached to the back to enable it to be displayed. The whole thing is then clipped into a circular wooden frame (which I removed to enable scanning).
Curiously, stamped into the metal back it says: COLUMBIA MEDALLION STUDIO CHICAGO. However, Margaret Beaty was from Caldbeck, Cumbria, and I have found no reference to the family leaving the UK (or, in fact, Caldbeck!) Perhaps the metal contraption was produced in the USA and used by photographers in the UK? (Seems strange though as there's no reason why such a simple device couldn't have been manufactured locally).
Bestest,
Paula.