Mitsumi standard type 2 tactile switches
For years there has been distinct uncertainty about the nature of the Mitsumi standard mechanical type 2 switches used in the Apple Extended Keyboard II (AEKII). Mitsumi mechanical switches appeared to be divided into two types: a vertical return spring giving a linear feel and an underslung return spring stretched over a nub giving a tactile feel. The AEKII switches have a vertical return spring, and so were always taken to be linear. Reports of light tactility was always taken to be an unintentional characteristic.
Having purchased some Apple service parts (922-0005) from Retrotechnology, I can report that they are in fact a tactile switch!
There are at least two designs of tactile mechanism from Mitsumi. One design uses an underslung return spring; this is the design that everyone is familiar with, used in some Amiga keyboards and in the miniature mechanical switches used by the likes of Devlin and Chicony.
The second design uses a step in the actuator leaf; this design is used in the AEKII’s switches. This step is subtle enough to have been overlooked all this time. Illustrated below are all three known designs:
On the left is a type 1 switch, which is linear. In the centre is a type 2 switch with an underslung spring, and on the right is a type 2 switch with a stepped leaf. Please note that the centre switch is an anomalous type with a different shape of actuator leaf.
The following images show a direct comparison between the linear (left/front) and tactile (right/rear) switchplates:
The linear switchplate is from a type 1 switch and the tactile switchplate is from a type 2 switch; the only difference is the step in the leaf, and the switchplates are interchangeable. Having swapped just the switchplates between a type 1 and type 2 switch, the tactility was transferred to the type 1 switch. It is likely that the reduction in shell size was for DIN compliance, as the change mirrors that of Alps SKCC to SKCL.
Both designs are found to be very tactile when a loose switch is pressed with a finger; in fact, out of NOS examples of Alps SKCMBB (cream damped), SMK Alps mount tactile and Mitsumi type 2 tactile (all purchased from Retrotechnology), the Mitsumi switch is the most tactile switch when tested loose. However, it seems that both the Mitsumi designs suffer from very poor tactility during actual typing; this has been confirmed to be the case with the miniature switches by at least two people at Deskthority, and it also appears to be the case with the stepped leaf variety based on machine translations of Japanese collector websites. No word has been given yet on whether the underslung spring design worked successfully in the full-size switches used in the Amiga.
The switchplate can be colourless or turquoise; examples from 1991 posted by rzw and MouseFan show a colourless switchplate, but mine are turquoise: