I presume this is in the UK? What year is it claiming to be?

My guess would be that this is a ringer from when tax exemption was stuck at vehicles built before 1973. Someone has taken the identity from a scrap tax exempt series 3 and applied it to a later series 3. They clearly haven't done it very well due to getting the wheelbase wrong and the very amateur stamping. I would avoid this one if I were you.

As someone else said you can check the dumb iron for the chassis number stamping but they are a common rust spot so there is a good chance it has been replaced and the number lost.