You do not need to write to FinCEN for a backfiling determination. Call the Helpline and tell them your problem. If they decide a backfiling determination is necessary they will send you a form. (The form is not available from their web site; they want to talk to you first.)
You will complete the form and send it in with a cover letter detailing the circumstances and I imagine that's where they will want you to describe your difficulties in estimating the number of paper forms involved and the amount of research time it would take to identify them.
My opinion is that an error in a DL number is material, but it's hard to comprehend why they would want you to amend all of the forms.
If you don't mind, please report back when you complete the process. This has always been a sort of a shadowy area of compliance and the IRS was completely unpredictable in its directives. I am told that FinCEN is more liberal with its determinations and that they actually have set, but unpublished, standards for when corrections are required! With the IRS it was a case by case hip shot.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.