Hi, Abe. If I understand you correctly, you are saying that you want to be able to look at the completed project from xx months ago and see how many of the subtasks were completed. Is that right?
When a project recurrs, its completed subtasks are either left in completed status or reset to uncompleted, depending on the selections made in the advanced options in the task recurrence window. You can force the repeating parent to leave behind a trail of completed copies by clearing the checkmark for "do not create a completed copy of this task on recurring" but that's only for the parent, not the subtasks.
If I wanted to do what you are doing, instead of a repeating project I would have a folder with 25 repeating subtasks, each of which would have the "do not create . . ." checkbox cleared. At any time the folder will contain all remaining subtasks for the current month and all completed subtasks for this and all previous months. The folder could quickly get pretty full so I would also recommend an aggressive aschedule of archiving.
-Dwight