There should be no differences. As the cars went from series 1 to series 2, the 1.5 cars were still series 1 bodies. The series 2 stuff was added slowly - but mostly the bolt-on stuff. The only thing I can think of might be the door latch hardware, but my early 1.5 still had the series 1 latches. Make sure you are comparing a real '66 body. When the 4.2 engine/trans was added, they were still using up the 3.8 bodies and adapted other 4.2 parts to fit the earlier tub. An easy check is to look at the access holes in the firewall for the pedal box. The early 4.2's still used 3 rectangle holes (using up the 3.8 bodies) and the later series 1 4.2 cars had only 2 (one is large). If both bodies look the same there, you're in business.