The Lotus 23C is the developmental evolution of the 23B. The claimed reason for the updates was to allow the car to be more competitive against ever developing opposition and in typical Chapman style, getting more out of a four-year-old design meant clever thinking rather than expensive re-engineering, hence the mods are relatively few.
Essentially, Lotus took the suspension, brakes and wheel/tire combinations from their Formula 2/3 cars of that year and fitted them to the 23B chassis. Visually, the cars changed too, as, to accommodate the wide rubber, flared arches were grafted on the 23B’s svelte glass-fiber bodywork.
While they might be simple, the updates work, not only does the revised suspension and far greater rubber area allow for much increased cornering speeds, the six-spoke wheels allow the new brakes to cool far more effectively as, while the magnesium wobbly-web wheels on the 23B look gorgeous, after a few laps you can’t touch the rims such is the amount of brake heat trapped by them.
Research points to there having been six factory 23Cs built (out of a total of 131 for all of the 23 variants), most if not all having been supplied to the USA, making them so rare that many of the often referred to books on Lotus fail to mention the 23C at all.