I think you could argue, to only have a lap belt , fixalot would know about that.
Your pre 72 a lot depends on who does your blue slip and their knowledge of the laws.
Even though you say your car is a 71 model if it was manufactured in 1970 , like a lot of 71 models were, by Australian law the car is a 1970 model .if manufactured in 1970.
This is an important fact when you are arguing your point at registration time.
Bfit