Original: They were the third variety of British Rail's then-standard 1972 design for suburban EMUs, which eventually encompassed 755 vehicles and five classes (Class 313\/314\/315 \/O/ 507\/508).
Simplified: They were the second variety of British Rail's then-standard 1972 design for suburban EMUs, which eventually encompassed 755 vehicles and five Classes (Class 313\/314\/315 \/O/ 507\/508).