This is a 1U (18.2mm) "low profile" flat keycap for MX switches or Kailh Box switches.
I mention "low profile" because it sits just above the stem of the swich and not at the height of the top of a standard keycap, in other words, the stem is 3.6mm and the keycap is 1mm.
This one can be tricky !
You CAN print it with the flat surface on contact with the bed (upside down), it will be faster, BUT you'll have to tweak your slicer to not overheat the stem, which is quite hard. (Or, you could print like ten a time so each stem has the time to cool down).
An easier option is to print it like it appears in the picture, with the stem touching the buildplate but you'll need supports. That's what I did. If you have an SLA printer it should work either way.
The stem may require some delicate post processing to remove excessive filament, but sometime it works right out of the printer...