as long as they have your name and address correct,you are nicked,and as toureman says,contest it if you want,but it will cost you a lot more in the long run.
If you can show that the registration number on the penalty notice relates to a Model T Ford (or any other vehicle where you're not required to wear a seat belt) you might be able to contest the penalty. Otherwise you stand no chance because, quite clearly, you were a driver or passenger (over 14 years of age) in a moving vehicle while not wearing a seat belt. That's an offence irrespective of the registration number of the vehicle.
they stopped you for a reason. Not wearing a seatbelt reduces the chance of survival in an accident. They were doing you a favour...why cant you see that???
Im sure you dont wanna be killed for not wearing a seatbelt, as much as the police dont wanna be scraping you off the road after an accident.