RAFI keycap mount
After buying some Siemens STB 11 switches, I tried putting a RAFI RS 74 M keycap onto them, and it simply fell off. From this, I concluded that the two switch families had only superficially similar keycap mounts.
Finally, I got around to taking actual measurements. My ca. 1984 RS 74 M switches have sliders approximately 5.0 × 5.1 mm, with the slot in the keycap around 5.1 × 5.2 mm. The Siemens STB 11 switch sliders were measuring around 4.9 × 5.0 mm, so why did the keycap fall off? They all appeared to be nominally 5 mm square, so it would stand to reason that the keycap should have fit.
The answer is that they are in fact not a friction fit. Most keycaps are held in place by friction, created by forcing the keycap around or inside a shaft. RAFI mount does not use friction; the clue is in the two loops on the underside of the keycap. These loops fit over small nubs at the base of the slider and lock the keycap in place. I simply had not pushed the keycap on far enough, since the ca. 0.1 mm difference gave them enough of a loose feel that I decided that they were too large.
RAFI keycaps are still a loose fit on Siemens switches, but once clicked onto the nubs, will stay on. They fit slightly better on the momentary switches I have (V42263-D11K6) than the changeover switches (V42263-D15K1); in fact, measuring the shaft of the latter is impossible with calipers as the changeover lever on the slider protrudes from the top of the switch and obstructs the calipers.
It would appear that “RAFI” (or “Rafi”) keycap mount is a square post mount that is nominally 5 mm square. There are two nubs, on opposite sides of the slider. Taking measurements from the Siemens switches (for which I have no keycaps) the nubs are 3 mm from the top of the slider, around 0.9 mm wide, 0.8 mm tall and protrude by 0.4 mm; at this size, and considering the shape of the slider, measuring them is fairly difficult. Removing a RAFI keycap takes a fair amount of effort, while removing a RAFI keycap from a Siemens switch takes nearly no effort at all, suggesting that the nub or keycap dimensions also differ between brands.
It does not seem that there is the same level of interchangeability as found with Cherry MX and Alps mounts, which seems a little odd considering that the designs are almost identical. I cannot measure how far the RAFI nub is from the top of the slider without taking a switch apart, and they are very fragile.
I do not possess any ITW or Devlin magnetic valve switches, so I cannot say whether those also accept RAFI keycaps and vice versa, although the design is visually very similar. With German flat-profile keyboards, the orientation of the slider nubs isn’t relevant (and varies between keyboards), but with Devlin’s contoured keycaps, orientation is relevant, and ITW/Devlin switches have the nubs at the front and back. The nubs also appear to be larger than those on RAFI RS 74 M switches.
Since three switch families appear to share this keycap mount, why am I attributing the design to RAFI?
The introduction dates of two of the families have now been identified. RAFI RS 74 M and RS 76 M were introduced in 1975. Siemens Siemens STB 11 was introduced in 1980, so STB 11 is tentatively discounted as a contender.
No confirmed dates are known for ITW/Devlin magnetic valve of any flavour, although patent US4017850 exists for the original design, which was filed in 1976. The intermediate size switch, whose mount resembles Micro Switch more than it does RAFI, has been found in a 1981 Sperry-Univac keyboard, so it would seem likely that the low-profile switch was introduced at some point in the 1980s.
There is no proof that RAFI produced the first keycaps with this mount, but they seem to be the oldest ones that we presently know about, so for now, RAFI keycap mount it is.