Weavers' Monograms.

By an edict of Charles V in 1544, each tapissier throughout the Netherlands, who manufactured, or caused a tapestry to be made, was compelled to weave into the border his own personal mark—generally a monogram— or his name, as well as such devices as his town might order.

  In Brussels they were required to weave into the galloon of each piece of greater area than six ells a red shield supported by two Bs. That stood for Brussels (the town) and Brabant (the provence). Look here for many examples.
It became a logo for quality. Several unscrupulous weavers in Antwerp and Italy added this BB sign to upgrade their product. Weavers in Antwerp, who go caught received harsh sentences.

Because the edges of the tapestry in the Ringling Museum have had some wear and tear over the centuries, the sign looks like two Rs, but originally looked like the monogram above.

Below are some of the more well known monograms.

Also, read Francoise Hack's paper on Tapestry Workshops here.