Have you clairvoyance skills? Or brandy new studies published about this matter? Have you not red the papers about the dates of "baby booms" for Y-R1a and Y-R1b? Toda ng ty percentages are not everytime a good picture of past percentage, as you should know.
To date, I confess the samples are still tiny, but we don't have too much Y-R1b (whatever the subclade) among the Atlantic megalithic societies studied to date, except if you can produce new papers? In my current knowledge, the Y-I2a2 + some Y-I2a1 haplos seem having taken the strong side upon the neolithical Y-G2a in the megalithic cultures, not Y-R1b for I know... I made this hypothesis about a R1b-megalithic culture on Atlantic: I don't abandon it completely but I strongly doubt; at least, if some R1b clan could have taken part in these western megalithic cultures, it would have taken place relatively late and in North Europe firstable; I had thought in the Long Barrows by instance but it seems R1b came in force at BB times only (what is not to say that FIRST southern BBs were by force Y-R1b people).
Maybe you have some new clues?