I did refer to some comments that I have read. (over several years) that a few immigration officers have ask for financial proof over and above the Income Affidavit - this would have been an origination bank or funding source. Maybe this practice has died out as I have not read any like those I refer to in a year or more.
But I do wonder if there have been forgeries of this affidavit and thus the MFA is checking Embassy staff signatures as well or maybe calling the U. S. Embassy to see if this document was in fact processed for the person presenting it.
The U. S. Embassy clerk does ask you if what you stated is true and warn you about making a false statement. But they do not check your income in any manner and do not certify it. They verify your ID with your passport. It is nothing more than a typical notarized statement. I've done this 3 times and know how it works.
You download the form from the U. S. Embassy Bkk Website. Or get one at the Embassy Consular / Notary services as you go in. You put the monthly amount of money you receive from America from any source in U. S. Dollars equivalent to $65,000. Check the Thai Baht / USD exchange rate. You then swear this is true. The document often called an Embassy Income Affidavit is given a Raised Seal. Most Thai Immigration offices accept this document as your proof of income. Some few want bank records to prove your monthly deposit. Getting this stamped by some Thai foreign ministry IMO is just flat wrong - bad rumor.