KBK update for Tuesday, 25th February
Research has already shown that Micro Switch SW began life with an older plunger design moulded from grey plastic. These older switches can be seen in advertisements, but they have not been otherwise encountered. All discovered SW Series switches had model numbers from 1SW11 upwards; however, model number 1SW1-R can be found on various part listing sites, albeit without any photographs or other evidence. It stood to reason that 1SW11 upwards was the circa 1970 redesign (with black plungers) and 1SW1 upwards was the original 1968 design (with grey plungers).
Finally, visual proof of early 1SW1 has arrived, in the form of a “3D model” of an unidentifiable SW Series keyboard, manufactured around 1969 judging by the dates on the switches. The switches are (surprisingly, for a keyboard that old) marked with their part numbers, with all visible markings being of model 1SW1 from week 26 of 1969. This is the oldest SW Series keyboard found to date. The model label is present but the stamped inscription is either absent or missing from the model; the only visible identifying code is that of the PCB, catalogue listing SW-10033.
This is not proof that 1SW1 was the older-style switch; photos of the next-oldest known keyboard do not appear to show whether those switches were stamped or not, and thus we cannot determine yet whether this is when the part numbers changed. The part number change could have instead corresponded to a change of some other characteristic, such as a redesign of the Hall sensor. As always, more evidence is still required.