If you perform a Google search and can't find them, they're just not there.
These keycaps are made and marketed towards gamers. People that extensively use the left side of their keyboard to play video games. Heavy wear on movement keys (WASD), reload (R), abilities (E and left shift).
The rubber provides "maximum grip" for high intensity, crucial moments.
Tai-Hao is tapping into a market and it seems that they don't think that a full board of these keys is necessary.