The Form 3520
/3520A are considered information returns. These are not considered to be associated with your actual tax return & OIC. The penalty can be assessed if you were liable to file the 3520/3520A and did not file them. I do not think there is a statute on these penalties, so yes, they can go back and assess them for that long ago.
The IRS may have made a mistake, so I suggest you wait and see what they tell you since you said you already replied to them and told them these forms were not applicable to you.
If they do not abate the penalty, then I would suggest you ask them for abatement under reasonable cause: No penalties will be imposed if the
taxpayer can demonstrate that the failure to comply was
due to reasonable cause and not willful neglect.
I am not versed in foreign
tax issues, so if you have any questions about the rules
on F. 3520/3520A, I'm afraid I'll have you ask someone else.
Thanks for your questions and good luck!