Matchbox Regular wheel cars were the ones that introduced me to toy cars as a child (thanks Dad and Mick!) and have formed a large part of my collection for many years.
The Matchbox cars in the 1 to 75 range started with the A series boxes for the first 7 models only. The B series of boxes were introduced and eventually encompassed all 75 models. Commonly referred to now as Regular wheels due to them having thick steel axles in the style most common up until 1970 or so when Mattel Hotwheels had taken control of the market.
Matchbox issued these models in various box styles over the years and my collection is grouped into those box styles from A to F.