The forged Mersons for French colonies & the French metropole have the same forgery characteristics.Only the upper and lower(Republique-Francaise) tablet with the colony name or for France metropole,'postes' changed.
The 5 fr Merson was used as model by Forbin (1912)which lead the way for forgeries of all the Merson types.The 5fr was expensive and at one point very difficult to find in post offices in France.There was a strong demand by collectors for used 5fr values.At the time forging mint stamps and reselling was severly punished under law.Forging a stamp and applying a forged cancel was not considered as an aggravated case under the penal code.When forgers examined the genuine Merson they noticed lines here and there which seemed as printing annomalies and they corrected them (photogravure).Those corrections were signatures left by the forgers.The lines and shading "annomalies " were in fact in the genuine matrice !
Forgeries (without a cancel)from the early period are rare.
There are other types of forgeries but they are more obvious to detect.
Looking at your pic your stamp looks genuine IMO.
Below a Forbin forgery:
e bay 15euros
delcampe 39,99 euros !
Genuine above,cupram stamp below (pic not clear enough for all detals).The post mark covers certain details around the shield.