I think what you may be looking for is a publication from ATF, referred to as ATF Publication 5300.4 revised September 2014 or Federal Firearms Regulations Reference Guide that is available online from the ATF website.
Under Title 18 United States Code Chapter 44 "The Gun Control Act of 1968," Section 921, paragraph 16 wherein defined the term "antique firearm" is given to mean,
(A) any firearm (including any firearm with a matchlock, flintlock, percussion cap or similar type of ignition system) manufactured in or before 1898; OR
(B) any replica of any firearm described in subparagraph A if such replica....
(i) IS NOT designed or redesigned for using rim fire or conventional centerfire fixed ammunition, OR
(ii) uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States AND which is not readily available in ordinary channels of commercial trade.
This is the portion of the 1968 Gun Control Act that you might be mistaken for a letter or ruling by ATF. You may want download a copy of this publication to see if it contains any new or amended statutes enacted since the 2005 Edition as well as updated regulations and rulings by ATF. Most all FFL license holders are provided with a copy as well as a list of all the exempted firearms under the Curio & Relic classification of firearms, also available from ATF.
For those who may be under the impression that this was a recent ruling simply based on whether .56-50 fixed CF ammo was commercially available again, this has been federal law now for over 50 years. One need only to glance through the 1969 Gun Digest to read about Navy Arms introducing a new Model 66 Yellow Boy lever action rifle in .22 rimfire and .38 Special, to realize why this statute reads the way it does. In the late 1950s, you had besides all the modern and antique firearms dealers, Navy Arms (established 1956) and also Centennial Arms (1959) formerly Mars Equipment Company.
Gun Digest (1969) reported: "the Model 66 carbine retails for $119.95... In addition, the same 66 is now offered in .38 Special caliber, the price is $139.95, and Val Forgett tells me he'll soon have a new cartridge for it. These, soft point hollow points made by RWS in Germany, will exceed 357 Magnum ballistics in the 18" barrel of the Yellowboy."