The horizontal column location is critical for correctly decoding the numbers. There are 7 items possible with the first item listed directly below the first letter of the rear axle code, or column number 15 and the column number is how they will be designated below. 

C-15 (I) ~ Power Brake and/or Power Steering Option Code:
  1 = Power steering only [RPO N40]
  2 = Power brakes only [RPO J50]
  3 = Power steering and power brakes [RPO N40 & J50]

C-16 (J) ~ Radio-Clock Group Code:
  1 = Radio-Clock

C-17 (K) ~ Radio Option Code:
  3 = Push button AM radio [RPO U63]
  4 = Push button AM radio with rear speaker [RPO U63/U80]
  5 = AM/FM push button radio [RPO U69]
  6 = AM/FM push button radio with rear speaker [RPO U69/U80]

C-18 (L) ~ Disc Brake Option Code:
  1 = Special suspension [RPO F40]
  3 = Disc brakes [RPO J52]
  4 = Special suspension and disc brakes [RPO F40/J52]

C-19 (M) ~ Air Conditioning Option Code:
  1 = Four-Season Air Conditioning [RPO C60]
  4 = Heater delete [RPO C48]

C-20 (N) ~ Power Window Equipment Option Code:
  1 = Power windows [RPO A31]
  2 = Power tailgate window [RPO A33]
  3 = Power windows and power tailgate window [RPO A31/A33]
  4 = Door edge guards [RPO B93]*
  5 = Power windows with door edge guards [RPO A31/B93]
  6 = Power tailgate window with door edge guards [RPO A33/B93]
  7 = Power windows, power tail gate and door edge guards [RPO A31/A33/B93]*

C-21 (O) ~ Power Seat Equipment Option Code:
  1 = Electric seat 4-way bench [RPO A41]

*Note: Several Fremont and Atlanta warranty cards have been seen with the numbers "4" and/or "7" in the C-20 position. These codes are listed in one reference, MSA-1 Chevy Cowl Tag Code Guide, but not in other reference material found so far.