So, I sort of vaguely know this from books but not direct experience.
There were a handful of firms in London, all of them in St. James, I think, that would get barrels direct from chateaux and bottle the wine themselves with their own labels. This is described in various novels, particularly Trollope.
Occasionally--VERY occasionally--I see wines on restaurant lists that marked as "Ch. Margeaux, bottled by Barton & Gustier" or some such.
But 1966 is very late for this...