Are these the row numbers from left to right in the profile picture?
R0, R1, R2, R3, R4, R4
With a very small height adjustment, R3 could be exactly the same tool as R2, just turn the legends upside down.
thats right what you said,
the keycaps from left to right can understand as from up to down on the keyboard,
can be see as "R0, R1, R2, R3, R4, R4" or "R5, R4, R3, R2, R1, R1" ,
and the R3 nearly the same with R4 you mentioned is the biggest upgrate in this keycap, the change makes typing more confortable and could quickly suits in this keycap profile when you use and touch the keycapu. It is a little change of the spherical keycap but big experience change in using.