RD 875688 was registered between October 1954 and September 1955. It is in the archive register BT53/213, according to Angela Bowey, who shows one in Bagley Glass 3rd Edition, p95, that was displayed at the Knottingley Civic Society's Knottingley Glass Exhibition in 2010. I have one of these with the RD number too, but haven't got round to taking pics for the RD album yet. I don't know if Angela has details from the registers at the NatArchives, so it's worth adding to the Lookup topic methinks.
NB This isn't proof that they were made at Bagley, just evidence that they were probably made by a glassworks in the Knottingley area.