Very nice, love those 1950 models. Not sure that a letter would tell you much more than which distributor it went to and exactly when; you might get lucky though. I have three of the pre-24 targets in the S14XXXX range; one was shipped April 21, 1955 to Higgenbothan-Pearlstone Hardware Co., Dallas, TX. Another to Rex Firearms in NYC on July 20, 1955 and the third to Matthews & Boucher and Company in Rochester NY on December 23, 1955. Great find, enjoy it.
Also have a couple of the 4 inch 1950s, one only a couple hundred SNs away form yours went to John Jovino in NYC also in 1955. The second one, earlier around S96XXX I haven't lettered yet.
Jeff
SWCA #1457