At some point of 17th century Catalan merchant ships started using white ensigns with a centered Coat of Arms of Barcelona or Catalonia, rather than the traditional yellow-and-red striped flags of earlier times (to let them look alike to other Spanish Empire flags perhaps?).
When in 1640 Catalonia shortly seceded from Spain, a red fleur-de-lis was added to each corner of flags, to symbolize the alliance to France.
Source: Francisco Manuel de Melo, «Historia de los movimientos, separación y guerra de Cataluña», Madrid 1645
Sizes: ensign W12 x L18 mm, mast flags W8 x L12 mm, jack W6 x L9 mm, pennant W4 x L48 mm.