Now that I have the full s/n, I checked the completed s/n lookup database. The two closest are 55569 shipped in June '64 and 55985 shipped in Oct '64. The Oct pistol is only 49 ahead of mine, so my guess would be Sept or Oct '64…earlier than I thought…Cool! I requested a ship date, so we'll get to see how close I am.
I know S&W s/n's are notoriously non-sequential, but from the 20 or so I checked, I think there were three that were out of sequence.