At first sight that looked just like mine, bought new in the mid sixties. It is not. Just out of interest on the side you show in 311_B mine has 'MADE IN ENGLAND', and on the other side, 311_A, the text is 'RECORD No. 311'.
Whether the one you show is earlier, or later, I'm afraid I cannot say. But I have found it a weighty useful plane.
xy