Here's one for the razor dating experts. I have a rather nice (Made in England) slim twist, The Knack, with a blue handle. According to official Gillette records, The Knacks were manufactured from 1966 to 1975, but this one is clearly marked with the code Z4 which I believe is the date code for both 1954 and 1979, both dates being outwith the limits of manufacture.I have the US equivalent of the Knack which is a slim twist with a black handle, dated P4 = 1970 which is fine (made from 1966 to 1987). Can anyone shed some light on this coding on the Knack?