Information on the activities of North Netherlandish panel makers is scant. Some evidence on one of these anonymous panel makers is presented below. It will become obvious that a focused research into the organisation and activities of panel makers in the Seven Provinces is urgent. The forthcoming Marks on Art database at the RKD will rely on this. 
A mark found several times on the reverse of North Netherlandish panels may be interpreted as composed of the cipher ‘4’ with below the two letters ‘M’ on top of each other: 4/M/M.
Artists using panels with this mark stamped or branded into the reverse are Herman Saftleven II (Christ Preaching from a Boat, dated 1642). Also the Rotterdam artist Jacob Lois (c. 1620-1676) painted on a panel marked with an identical monogram when he in 1647 signed his Baptism of Christ. Ten years earlier his city companion Jan Daemen Cool (1589-1660) painted a Portrait of a 36-year-old lady in 1632 on a 4/M/M-panel, so far the the panel maker’s earliest.
Also Simon Jacobsz de Vlieger (1600/1601-1653), Wybrand de Geest (1592-1661), Hendrick van Anthonissen (1605-1656), and Barend Avercamp (1612-1679) painted on boards provided by the same panel maker.
Finally, Bartholomeus van der Helst, born in Haarlem 1613 but moved to Amsterdam in 1636 where he practiced his art until his death in 1670. Here he portrayed Samuel van Lansbergen and his wife Maria Pietersdr. de Leest. Both paintings are signed and dated ‘1646’, and the panel used for the man is showing the panel maker’s monogram struck four times into the back, on the panel of his wife it’s only found once.  Another Portrait of an elderly lady with a ruff, aged 62 was painted by van der Helst in 1648, and the panel equally marked 4/M/M.
From the above it becomes apparent that all the artists are practising their art i the Northern Netherlands. Their panels are all machine sawn (contrary to the large majority of the Flemish panels that remained hand sawn), and were painted within a limited range of years from 1632 through 1648. However, the panels were not painted by artists working in the same city or region but as far apart as Leeuwarden in the north via Kampen, Zutphen, Amsterdam, and Utrecht down to Rotterdam in the south.
Was the panel maker with the enigmatic 4/M/M-monogram a Fleming that had fled to the North?
One such was the Antwerp frame maker Reynier Roovaert who in 1638 got permission to open a shop in Amsterdam where he would sell ‘all sorts of frames and panels, both of oak and other sorts of wood, including standard-sizes, to painters and other customers’.  We do not know if Roovaert issued a house-mark on his frames or panels, but we do know of several makers’ monograms on North Netherlandish panels that so far have no name associated to them. 
Further research into the frame and panel makers’ of the Seven Provinces would clarify their activities and practices and in this way add significant information about the complexity of the huge art market of the Netherlands in the seventeenth century.
 New MARKS ON ART database under construction. For more see here.
 Both oil on panel, 68 × 58 cm. Rijksmuseum, inv. SK-A-143, SK-A-144. The panel of the woman is dendrochronologically dated to after 1639 (P. Klein). Thanks to G. Tauber and M. van den Bichelaer, Rijksmuseum Amsterdam, for drawing my attention to the marks; Oct.-Dec. 2019.
 ’alderley soorten van lysten ende panelen te mogen maken, zoo van Eycken als ander hout ende alsoo dosijnwerck aen de schilders ende anderen te vercopen’, J.G. van Dillen, Bronnen tot de geschiedenis van het bedrijfsleven en het gildewezen van Amsterdam, 3 dln., Den Haag 1929, 1933 en 1974, pp. 176-177, nr. 340. See also P.J.J. van Thiel & C.J. de Bruyn Kops, Prijst de Lijst, De Holælandse schilderijenlijst in de seventiende eeuw, Rijksmuseum 1984.
 You may download my poster Documenting North Netherlandish 17th Century Panel Makers’ House Marks (ICOM-CC 2014) here.