Forgeries of Early Mexico

While putting together some Mexico listings this past week I came across a grouping of relatively common issues from 1864, depicting Hidalgo... often called the "Juarez Issue".  It is a handsome series, engraved, and perforated 12.  The vast majority exist unused and without overprints and are thus relatively inexpensive.  There are numerous forged cancellations.  What caught my eye though, as I was putting together the lots, was that one of them "didn't look right." Pictured below are two 4 Real Brown side by side.  See how many differences you can come up with.  First, figure out which one is the genuine and which is the forgery.