What's the difference??
I use "an" with 'istoric as well.
I have to look this up ALL THE TIME.
Without disagreeing with the example at all, here is usage note that may help you from the American Heritage Dictionary:
Usage Note: Historic and historical have different usages, though their senses overlap. Historic refers to what is important in history: the historic first voyage to the moon. It is also used of what is famous or interesting because of its association with persons or events in history: a historic house. Historical refers to whatever existed in the past, whether regarded as important or not: a minor historical character. Historical also refers to anything concerned with history or the study of the past: a historical novel; historical discoveries. While these distinctions are useful, these words are often used interchangeably, as in historic times or historical times.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.