all my instincts say that that stopper was made at Whitefriars. I have matched those canes with canes in Whitefriars paperweights from the late 60s and early 70s. However the stock spirit decanter with a millefiori stopper CM637 didn't appear in the catalogues until 1972 when the Whitefriar date cane was well established and appears in decanter stoppers (probably because there is no millefiori in the base of the decanter). Also it was facetted 5+1.
Your stopper looks a lot more like an Ink bottle stopper which of course, had been made for many years before the millefiori decanters appeared. All my ink bottle stoppers do not have a Whitefriar cane in them as the Whitefriar cane appears in the millefiori base. The ink bottle stoppers are round (like a plain cut paperweight) as this one. Plus I do not recognise the cut on that decanter. It does not have the look of Whitefriars.
It would be good to know the dimensions of the stopper as that would settle whether it is an ink bottle stopper that has been made to fit a decanter or if it was always designed for this or another decanter. However my guess would be that it is a Whitefriars ink bottle stopper fitted to a gash decanter.