The plywood may not be old siding; it's probably just the wall sheathing. Could you tell Mike? What was visible from the attic, if accessible? If the plywood "siding" was original, then it was improperly installed since it's too close to the roof line. Every installation detail I have ever seen indicates that step flashing needs to be installed in between the siding and wall sheathing. If there is step flashing run in behind the plywood, then it's less of an issue, but still wrong. The plywood is most likely going to rot out over time.