That would be the body number at the plant your car was built at, that's a low number so it was probably built in Aug or Sept 1957. Vern Franz at New Jersey Late Greats tracks all that stuff or you will find him on ChevyTalk.
Ray,
That was the 2,320th Impala 2 door hardtop body built at the Van Nuys (not VIN) plant in La. The Vin number would give you a better estimate of when the car was assembled. I don't have any data on end of month VIN number from production like the 55-57 chevys so that will make it a bit more difficult. Production started in Sept 57. Probably the best way to estimate build date is to gather up all the dates from the known origianl dated components and work from there. :