Hi @salvor1,
The R2 version was released in July 2018 (see the Revision History in the Technical Details of the product page for other revision details).
For quick reference,
R1:
R2:
R3/R4:
(although note that R2/R3/R4 use the same electronics, so it’s not as relevant to keep a set of the same ones together, whereas R1 had different light levels to the others)