PLEASE HELP! found #s on inner door/floor frame , dont know what they are!!!

Found a 7 digit number/letter combination on inner door frame that doesnt match vin /title any of that.... The vin# on the door matches title, but this number I found on floor is scaring me, cant find what it is! We just bought the car 2 weeks ago, its a 74 Cuda. Anyone have ideas?