“SEVENTEEN MANUFACTURING ESTABLISHMENTS were in full operation in London, with a force of four hundred men  devoted to the production of false and forged Assignats. The success and the extent of the labor may be judged by the quantity and value they represented. In the month of May, 1795, it was found that there were in circulation from 12,000,000,000, to 15,000,000,000 francs of forged Assignats, which were so exact in form, appearance. , texture, and design, as to defy detection, except by the most minute examination and exact knowledge of the secret signs by which the initiated were taught to distinguish them.“ (p. 33)

Also see footnote 1. And while we are at it - a second source

“While, as we have seen, such enormous sums, rising from twenty to forty thousand millions of francs in paper, were put in circulation by the successive governments of the Revolution, enormous sums had been set afloat in counterfeits by criminals and by the enemies of France. These came not only from various parts of the French Republic but from nearly all the surrounding nations, the main source being London. Thence it was that Count Joseph de Puisaye sent off cargoes of false paper, excellently engraved and printed, through ports in Brittany and other disaffected parts of France. One seizure by General Hoche was declared by him to exceed in nominal value ten thousand millions of francs. With the exception of a few of these issues, detection was exceedingly difficult, even for experts; for the vast majority of the people it was impossible.” pp. 77-78