+1 with a two additional comments
1. Some parts will have different versions to account for different colors (steering wheels, for example)
2. At some places I have worked, and worked with, as an engineer, the decision to use "A1" instead of "B" to denote a revision can be based on cost, driven from requirements. For example, going from A to A1 will take a relatively low level of additional testing and signatures, whereas for a full letter revision we might go back to square 1 for testing, which drives up cost.