@edw_kim - Even tough your stamp is not a postal forgery I am afraid that does not necessarily make it authentic. It might very well be authentic, but the argument that is not a postal forgery and hence it must be authentic, is not valid.
Please note the difference between
Postal forgeries: (made to defraud/cheat the postal system while the stamps were actually in use - these are often highly sought after and very valuable) or
Fakes/forgeries: made to deceive collectors and were produced years/decades later....mostly with minimal retail value today. Tough certain extremely well made forgeries made by Sperati and others might be even more valuable than the original stamp itself (despite the fact that they were also made to deceive collectors....its complicated!)
To complicate things, some stamps were re-distributed from the printers waste - and sold to collectors. To make them appear authentic, someone applied a fake postmark - and voila - it might appear to be a postally used authentic stamp. For your particular stamp in the OP, you stated yourself:
the cancellation has been studied further and the date reads 20th of feb 1867.Genova cancellation..forgery is out of the question.
Please consider the date of postal validity for these stamps, this was made available for you in my earlier post (page 2 in this thread) with information from the Sassone catalog. Then make your own conclusion.