Oak Series 400
Very little is known about Oak’s non-membrane keyboard switches.
Oak advertised (in Electronic Design magazine in 1973) a type of tall, transparent, mechanical switch as Series 400. The current owner of Oak Industries’ switch line reports that no details remain on the keyboard switches.
A switch matching this visual depiction showed up in a Honeywell 80-02 keyboard, from a Honeywell 5921 terminal.
Oak Series 400 is named in US patent 3924722 (but not in Google’s transcript at the time of writing, which is erroneous — download the PDF), an electronic typewriter patent from CPT Corp, filed in 1973, the same year as the advertisement.
US patent 3708635 depicts switches externally identical to Series 400, and internally identical to those in the Honeywell keyboard. This patent is moulded into the shell of a different design of switch.
There Oak switches in some unidentified Facit keyboard use the metal plate attachment depicted in figure 13 of the above patent.
Additional photos depicting Series 400 as well as the smaller type.