The following switches are known to exist, but practically nothing is known about them and few if any are still obtainable.
Dai-Ichi are or were a clone manufacturer.
These were listed by AliExpress seller harry ke, but by the time I came to order them, he was shipping out genuine Omron instead.
I salvaged this photograph from the AliExpress transaction snapshot for my order, as the current photograph on his store depicts the actual Omron switches that he carries.
This has subsequently been identified as Dai-Ichi CL-1145; I’ve yet to get my hands on a white one, but I have a number of the green ones.
Dai-Ichi Cherry MX clone
AliExpress seller gao xiaoqing depicts these.
Supposedly these can still be ordered, with an MOQ of 2000. Note the slider — yellow for clone again!
These switches can also be seen in a Micronorth KB-5161 (misreported as Chicony), this time with black stems.
Omron ball contact
This picture is from the same gao xiaoqing AliExpress item as the Dai-Ichi Cherry MX clones. These switches are reported to be no longer manufactured.
These are now identified as genuine Omron; the Omron branding on the base was not discovered until they reappeared for sale. These are presently termed “Omron ball contact” and are not Futaba clones: they accept different keycaps and the terminals are at 90° to the Futaba ML series design. Internally they are very similar to Tokai MM9 series, which was a most unexpected find.