Micro Switch SW and SN Series
Tentatively it appears that SW Series was the original Micro Switch keyboard series, introduced at the end of the 1960s. No literature has been recovered that names the series or lists any of the part numbers. What information exists does not give a series name.
SW Series switches examined to date are all open at the base: nothing holds the return spring inside the switch, and the Hall sensor is a slide-in fit. These all fit into sheet metal securing clips.
SN Series is documented by name, although so far it is only listed as a switch series. The packet for 201SN1B1 refers to the parts as 101SN and 201SN series switches; potentially 201SN denotes illuminated, based on the one confirmed type so far.
101SN appears to be identical to SW series (having not examined an illuminated SW Series switch). SN Series switches are fitted with a base, that is either glued in place, or secured very tightly, as it is held too tightly to remove with risking destruction of a switch type that costs €20/piece NOS.
The base notwithstanding, it is otherwise identical to SW, right down to the injection moulding marks. At present there is no way to determine how SW and SN differ, as there appears to be no fundamental difference.
SN switches have perhaps the strangest mounting method yet. They have two strips of double-sided tape on the bottom, covered in backing paper. These are designed to secure the switch to the PCB prior to soldering. With age, these strips have deteriorated, such that removing the backing paper pulls the tape off with it.