Hiya guys, got this Model M that some old professor discarded a couple of days ago, but it doesn't have a model sticker. Strangely, it has a part sticker behind the speaker grille that looks like a normal sticker, but it reads "1386715" which I can't find anything about on wikipedia or anywhere else.
It's a UK layout, grey label model with two-part keycaps and a detachable SDL cable. It doesn't appear to have the drainage channels. I'd open it up, but I don't have the proper nut driver for it.
Anyone have any idea what part it is? Pictures below.

