Today, despite widely available technology such as high-resolution scanners and printers, counterfeiting is more difficult than it was at the time of the Civil War, when it was estimated that one-third of all currency in circulation was counterfeit.
This is from a GMAT question. The book I have says that "it was" can not be omitted, since you can not compare "counterfeiting" with a time description.
In regular speech, one can omit the "it was". What I want to know is whether it is possible to omit according to "official" grammatical rules.