Drew;
You did not cite a source for your chart above but we British gun and rifle history buffs know it instantly as page 435 (Date Marks) of Nigel Brown's BRITISH GUNMAKERS. Vol 2. Brown's chart itself presents a conundrum. Did the Birmingham proof house make the same mistake that they made in 1952 again in 1977 and produce a incorrectly made date code stamp? Brown's chart above leads one to believe that the Birmingham proof house did that but I have not encountered anyone or any source confirming that mistake/error.
I suspect that it is a book printers error. If anyone knows why Brown illustrated this let me know.
Kindest Regards;
Stephen Howell
