De Zwaan, from the Netherlands, sells on 1 November 2017 as lot 4509 a "Northern European School, ca. 1600" portrait of a seated man, estimated at 1,500 to 2,000 Euro.
"... Reinius Hisp.
Et. S. Anno LXII"
The top line seems to be his name, Reinius (? A Latinised version of Rein or Reinier perhaps, if I read it correctly) an "Hisp." an abbreviation of Hispanicus or some other word indication a link with Spain. The second line simply says that he is 62 years old. I have so far not found any person who could be matched with this name.
Claes Moeyaert from 1647, or one by Johannes Vinck from 1614, or a third by Samuel Hofmann from 1616. The standard position of books as we usually know it is shown on many other paintings of the period, but this one is not unique, just peculiar.
And researching this painting and topic in the RKD, where all the above images are from, I suddenly came upon a work by Marten van Valckenborch from 1589: this very painting!
The work was in a private collection, but previously sold at Sotheby's London in 1978, and Sotheby's Amsterdam in 1988.
It has not only an attribution, but a better description of the inscription:
'1589. M / V V' (V V beneath the M).
Below, in lighter paint: '...) PLINIVS HISP. / AET.S. ANNO LXII'.
The resemblance in style is quite clear, so an attribution of the work for sale to Marten van Valckenborch seems reasonable.
As a signed Marten van Valckenborch, but far from his best work, it should be worth around 5,000 Euro.
UPDATE: sold for 7200 Euro, more than 3 times the highest estimate and even above my estimate!
UPDATE 2: now again sold at Dorotheum (24 April 2018) as a Marten van Valckenborch, where it fetched 31,250 Euro. A nice profit for whoever bought this a few months ago. Seems too much to me, but it shows the value of picking the right auction house, and even more importantly getting the right attribution (certainly in this case, where it was known all along, not some recent discovery).