This dolmen is not a real dolmen, not even a reconstructed one, but a "synthetic" dolmen. Judging by the numerous ancient postcards, the two stones (called "Peetje" and "Meetje", two diminutives of popular forenames) were originally much further apart than today.
I haven't found out so far when the monument was assembled. "Meetje" and "Peetje", unearthed and put closer to each other, were supplemented by a few other stones found or collected nearby (see site "Kluisberg", about 1 km away). The present dolmen cannot be older than ca. 1930, and could even be much more recent.
Something is not right. This message is just to keep things from messing up down the road