Stuart, it's the part machined backplates Andy is interested in which are then machined to suit whatever chuck is being used.
The premachined hole in a 280 backplate is 52mm dia and the studs which could always be redrilled are on a 75mm PCD (roughly measured and calculated) and I suppose you could also bore it out to fit your 290.
I'd be tempted to get a slice of 130mm EN3 for about £10 and machine that up to a nice fit on your spindle nose, put the £40 saved to something else