Thank you, Anne.
I had requested a detailed condition report on the glass, whether the glass was pressed or blown, confirmation of the RD number and its postion, and permission to save an image for use on GMB, but I am still waiting for a reply from the seller.
It could well be then, that the registration number relates to the frame rather than the glass (sorry if it's become largely a non-glass related post as a consequence), but it's still really odd that the number doesn't show up in searches at the NatArchives.
I've got a few 'oddities' and apparent ommisions regarding RD numbers (mostly of glass but also some metalwork and china) so I'll give the newNatArchives search function a whirl. Many thanks for mentioning it. With a bit of luck it may even reduce or negate my need for a copy of the Blue Book.