All factory 340 cars have a somewhat low production number. Add a 4 speed and the number gets even lower.
When you say "it looks like it came with a 340 4 speed", what are you using to make that determination?
Did you decode the vin? That will tell you flat out, if it was a factory 340 car.
The fender tag and/or build sheet will tell you if it was a 4 speed.
Remember, rare and valuable do not always go hand in hand.