There are several forgers of this stamp series but most apparently only produced one or just a few values so there is not much chance of finding them in your collection.
So I will only comment on the 4 main forgers, Fournier, Spiro, Foure and Mercier
George Foure
German philatelic merchant, good friends with the well known engraver Schilling from whom he acquired original plates of many rarities.
Release: Europe, early 20th Century
Printing: Photo-lithographed
Paper: Soft, yellowish wove
Cancel: 33 mm double ring with legend CORREIO GERADACORTE
Design Flaw: Background cross-hatching (burelage) is blotchy and coarse.
Louis-Henri Mercier
Louis-Henri Mercier sold all his stock to Fournier shortly after his bankruptcy
Lithographed on smooth, hard wove paper
Very good forgery, but paper is too thick
Design Flaws:
10r - Vertical stroke of "1" is slightly concave instead of straight
20r - Tip of "2" ends in a ball instead of a point
30r - Opening inside "O'' is too wide
60r - Ball at top of "6" is a full circle instead of a half circle
90r - Inner oval at top of "9" is too large
180r - Top of "8" inclines to right
300r - Left side of the middle "O" does not touch shadow of the "3"
600r - Opening inside last "O" is too large
Fournier/Spiro
The following were sold by Fournier but based on the 5X5 sheets and cancels were probably from Spiro. Fournier sold the complete 1850 & 1861 series
