That's the month/day code when your car's body left the paint shop at GM. The date code letter starts with "A" for the first month of production (I don't know when 81 starts) and "21" is the 21st of the month. Usually it took a "couple of days to a week" for the car to make it all the way down the production line after being painted.
Trim Code A25 started with VIN 400114 on August 22, 1980. With that B would equate to September 21, 1980. There is a little more involved than this...
B22 started September 22, 1980 and was Ser.#403177 My guess is your Ser.# would be a few less than 403177 to verify the trim code.
Also, FYI The trim tag code was normally installed between the second and third coats of paint, but depending on day of the week, holidays could me production one week and trim code the next. The MFD date was determined when the Certification sticker was applied, and this was done during the very end of the assembly.
The St. Louis plant averaged approximately 93 units a day during this period and went as high as 165 units per day by February 1981..
Sorry for getting so long winded here, but, Just happens to be one of my favorate year of Corvettes