Info needed on affidavit in lieu of EVL

[Amused]

I understand that USCIS (may) accept notarized affidavits from colleagues as EVLs, in cases where the letter is not obtainable on a company letterhead, because the company might not be existing any more. What is the USCIS acceptable standard for getting such letters from India? Will a Indian-notary signed affidavit do? Or, does it have to be in some other leagal format?

Thanks for all suggestions.