Prince Maurits or Frederik Hendrik?

At Doyle of New York, they sell on 20 May 2015 a "Portrait of Prince Maurits of Orange-Nassau" attributed to Michiel Jansz van Mierevelt, and estimated at $2,000 to $4,000.

However, Prince Maurits (pictured above) didn't resemble this portrait at all.

The depicted is most likely his half-brother and successor Prince Frederik Hendrik of Orange-Nassau, shown here in a Michiel van Mierevelt portrait from the Rijksmuseum

The one for sale is very similar (facially spoken) , but perhaps less well painted, so a "Workshop of" Mierevelt may be more appropriate. But it is a high quality picture, of a known royal sitter, one of the best I could find for him actually (apart from the one from the Rijksmuseum), and should be worth at least $10,000, even though it is very small (24 by 18 cm). UPDATE: sold for nearly $6,000, so well above the higher estimate but clearly below my estimate.

Looking further, the RKD learns me that the Paleis het Loo, a former princely residence in the Netherlands, owns a copy of this portrait, attributed as "After Mierevelt". Of similar dimensions as the one for sale (though octogonal), it is hard to tell from the black-and-white photograph which version is the best.

The RKD also shows a different version of the same portrait, which was in the art trade in the Netherlands in 2012, as After Mierevelt. They presume that some original unknown Mierevelt must be the basis for these portraits, but deducing that the one for sale now is that original may be too optimistic. But there can clearly be no doubt about the sitter in the portrait for sale!