Interesting link:

1949 Series One 80

According to the parts list they modified the canvas top three times .

1. 860001 to 8666000 ( strap fixing type )
2. 8666000 to 06105462 ( rope fixing type .. .for flat bows )
3. 06105463 onwards ( strap fixing type.. for curved bows )

I think the rope / strap thing is mainly refering to the rear curtain and how it is fixed to the rear bow. They also mention the export version as having side windows. I've seem some with a cotton gauze material instead of flexible clear plastic for the rear windows.