I'm unsure of the serial number sequence and dating.

In poking around online, It seems the "C" designation for the 340 is used on all barrel lengths as I've seen 340C's with 18.5, 20 and 22" barrels.

The following "Shooting Times" article indicates the 20" carbine was introduced in 1962 and dropped two years later but makes no mention of an 18.5" carbine? Seems the later "E" series carbines had 20" barrels and I've seen a Springfield 840 with 18.5" barrel. ???

http://www.shootingtimes.com/2010/09...0roots_200810/