Nice Van Mieris, but is it Willem or Frans?

Bamberger, from Germany, sells on 3 December 2016 as lot 922 a "Willem van Mieris, signed" scene of three men in a window, estimated at 3,000 Euro.  At 25 by 19 cm, it is a small painting.

While Willem (and the other members of the Mieris family ) are well-known for their images of people doing entertaining things in a woindow frame, these usually are more refined people in curved, stone windows; the work here is much more in the tradition of Van Ostade, showing the less restrained peasants. With the man in the upper left looking straight at us, and the two additional portraits (oval top right, and rectangular paper bottom right) of similar men, I wonder whether this isn't some kind of triple self-portrait painted for fun between the more formal, stiff genre scenes that filled much of his career.

When looking for comparable paintings, I noticed the above ones. The first one is by Willem van Mieris, and has a square "inner frame" and similar dimensions (20 by 16 cm). However, the style and theme is much more typical for Van Mieris. The second one, sold at Sotheby's in 1998, is again a bit closer wrt the composition, but the topic is the same stylized, rather boring Mieris work. The final one, for which I have only this engraving and not the original (which was probably lost a few hundred years ago), is by far the closest to the one for sale and evidence that Mieris indeed painted this kind of work. However, according to the RKD (where I found all three images), this one is originally by Frans Van Mieris I (1635-1681), not Willem van Mieris (1662-1747, son and pupil of Frans I). Not having seen the signature on the work for sale, I can't judge whether it supports Willem or Frans in this case.

It is hard to compare the estimate of this one with other paintings by Van Mieris. The image in the auktion catalogue doesn't really permit to judge whether it is an original or not: it is well painted, but I'm not sure it is really good enough to be by either Mieris. However, like I said, this may have been a rather swift divertimento between more tedious but lucrative genre paintings. If it is a real one, it should be worth 10,000 Euro (for a Willem) and close to 50,000 (for a Frans I).