This map shows high frequencies of this subclade in Southern/Central France as well as in Northern Italy.
R1b L51 seems absent from the Balkans and eastern Europe in general (except southern Poland).
Also, it is worth mentioning that R1b P312 main subclades(U152, M167,L21) have their highest variance in France.
So it looks like L51 formed in southern France (and not in eastern Europe) during the late Neolithic (Chasséen?) out of R1b L23 people who had arrived from Italy.
Richard Rocca explains it very well in this thread:
EDIT: the point is that L51 and its successor L11 spread out of Western Europe to central Europe and not the other way round which makes them unrelated to the Bronze age central European cultures (Unetice, Urnfields etc..).