MPEP 715.09 Seasonable Presentation:
Affidavits or declarations under 37 CFR 1.131 must be timely presented in order to be admitted. Affidavits and declarations submitted under 37 CFR 1.131 and other evidence traversing rejections are considered timely if submitted:
(C) after final rejection, but before or on the same date of filing an appeal, upon a showing of good and sufficient reasons why the affidavit or other evidence is necessary and was not earlier presented in compliance with 37 CFR 1.116(e).
Your good and sufficient reasons for entry of your 131 affidavit are that the examiner never used the reference your ante-dating until the final rejection, therefore you never had to submit a 131 affidavit.
If your FAOM was a typical A v. B rejection where both references A and B could not be antedated, and your first amendment caused the examiner to change the rejection to A v. C where C could be ante-dated by a 131 affidavit, you have the right under Rule 116(e) to submit the 131 affidavit and have it considered.
I had the same situation I describe above except the C reference was ante-dated by a certified copy of our foreign priority application. I submitted my after final response along with the certified translation. The examiner issued me an advisory claiming my response presented new issues that would require further consideration and/or search. I spoke with the examiner. Pointless. Called the SPE. He gave me the usual, "I'll take care of it" schpiel. Nothing happens. I leave some voice mails. Still nothing. Call the TC Director, politely and firmly express my extreme displeasure for the horrible way the case is being handled. Very apologetic call from the SPE an hour later. Case allowed.
Start calling some people. Start with the examiner (because once you work your way up to TC Director the first thing you're gonna be asked is, "Well, did you call the examiner? Did you talk to the SPE?" and you want to be able to say, "Yes, but it got me nowhere."