Well i have one of those, not the "stainless" drum type but the alloy drum ones. Going by Bill's information mine is a 1964 (456/64) ? I'm not a collector so i have NO credible information on these mic's. What i do have in front of me is a braille micrometer, its condition is what you'd expect for something made in 1964. If someone had gone to the trouble of making "knock-offs" they would certainly not be selling them for £35 each ? I haven't stripped it down but even the work involved in the gearing of the three tumblers must be quite something, for £35 ? No, in my opinion Bill's information may be right but may apply to the "stainless steel" drum ones. I suspect that the alloy ones were made by some company, not necessarily by M & W but perhaps some other company ? I'm not interested enough to become a detective but perhaps Bill could help.
Actually if Bill has a drawing showing the mechanism i think it would be very interesting, i don't feel confident enough to strip this one.
Anyway if i can help just shout ! if needed i can take some pictures of my one so you can "verify" details.
Mike Young