The FRT only helps if all the possible variants listed are classified as antiques. If there is any doubt as to the specific gun, only the letter will be definite proof. For example, if you have a Colt M1877 Thunderer in .41 LC it could be antique, restricted, or prohib, depending on the serial number and the barrel length. The FRT's don't usually list the serial number cut-off (some do), and unless you have the supporting documentation proving that your particular serial number falls into pre-1898 territory, the FRT would not help. Just today I had two inquiries about this - one was for a Colt SAA I sold last summer (with factory letter confirming age and caliber), and the other for a S&W New Model No. 3. In both cases the status was undeniably antique. The Colt is in .41 LC with the letter stating shipment in the early 1890's, and all variants for the S&W in .44 Russian are considered antique. No ambiguities there.
If any of my customers ever find that they are having difficulties obtaining an antique letter for a gun I sold them, I would either take it back for a full refund, or help them get the letter. I have not run into this problem yet.