I JUST GOT A 74 75/6 ABOUT 90% RESTORED. I HAVE OTHER OLDER BIKES FROM 60S 70S AND 80S. BUT MY FIRST BMW. WHAT IS THE DIFFERANCE BETWEEN A 75/5 AND /6 AND /7?? I WOULD LIKE TO LOOK AT THE OTHER BMW BIKES. THE 900 CC AND THE 1000 CC SIZE. THANKS DANIEL