Does anyone know what the 5-31B option is? It is on an early 66 (SS sights).

What gave you the idea there was a 5-31B option? Where did you read "5-31B?" You might be reading the assembly number that is stamped on the frame, yoke and back side of the side plate to keep those fitted parts together during the manufacturing process.

k22fan, the option was on the label on the box labeled "Special Features" where I understand S&W identifies options.