If you were lucky to be around when STAR KEY Industries, Inc. out of Brooklyn, N.Y. was around, you would agree their key blanks and numbering system were very friendly to the locksmith profession.
STAR was considered the innovators of the true EZ coding system, which was designed to easily identify key blanks by reference only to the numbering code.
It's numbering system typically consisted of three parts.
1 - The "prefix" which designated the number of pins in the lock.
2 - A lock manufacturer identification code.
3 - A keyway "suffix" which designates the keyway.
A 5CO1 fits a 5 pin Corbin lock and a 5CO1, 6CO1 (6 Pin), and a 7CO1 (7 Pin) all fit the same keyway but each work with a different amount of pins, 5, 6 & 7 respectively.
Back when cars used a 2 key system, one for the ignition and one for the doors or trunk, STAR used different prefixes such as "H" (hexagon shape of the key) for the ignition and "O" (oval shape of the key) for the doors or trunk. These key shapes were typically used by General Motors and sold by Briggs & Stratton at the time.
The STAR brass blanks were great for impressioning locks because they were soft enough to get great impression marks but still strong enough to withstand torque and wear. Impressioning marks are more visible on brass compared to a nickle plated key. The fact that these keys where both soft and hard at the same time, which sounds like an oxy-moron, made them the key of choice by many locksmiths. The traditional brass keys self lubricated the keyway with the brass to brass contact against the internal brass pins of the lock cylinder itself.
Certain keyways were also available as a modified form of a "universal" keyway, such as in General Motors vehicles. These universal key blanks allowed for a specially milled key to pass into a variety of keyways or sections.
Simply put, STAR was an industry STAR!