The Classic Velocity crew considers the postwar period from the late 1940s to the late 1970s to be an era. Why? Because it is a time when the engineers and the stylists often triumphed over sales and marketing, when the engineer's stopwatch beat the accountant's calculator. Is there any rationale to support why we might define this as an era? No, it is entirely arbitrary and capricious. It starts with recovery from World War II, and ends when chrome bumpers ended. Dictionary.com defines an era as "a period of time marked by distinctive character, events, etc". So there it is, we have defined and declared the Classic Velocity Era.
But wait, hold on a minute, what if there are eras within eras? what if eras overlap? What if we have defined an era in error? Take Porsche for example. Within the classic velocity era, Porsche clearly has a 356 era and an early 911 era. So is an era equivalent to a model run? What about the 914? BMW had Baroque Angels, the Isetta, and the fabulous 507 all overlapping in the late 1950s. These could not have been further apart in character or in customer. Do they each have an era, or do they belong to the same era? Does the air-cooled VW Beetle define an era lasting half a century? Audi was about 4 different companies during the Classic Velocity era. Are these sub-eras, defined by ownership? Auto Union, Volkswagen Audi, the NSU-merged Audi?
Periodically, you talk with friends who chop eras into very thin slices. They may say things like "The only true early 911s were the short wheelbase cars" or "The roundie 2002s are not the same as the squarelight cars", These fans of slivers would have a really hard time with something like the Paleolithic era which lasted a few million years! Mercedes attempts to sort out this mess by the use of chassis numbers. The W113, or the R107 are clear identifiers of a model era. But wait, they also refer to the era of the Fintails, which spans the W110, W111, and W112. The Ponton cars preceding them had even more chassis numbers included. Just when you thought it was safe...
And then there are eras defined by people. The Quandt family at BMW, Albert Roger at NSU, Ferry Porsche designs, the Bertone era, the Max Hoffman era, etc. What about the whole era of the "3 box design"? What about engine configurations like the flat four VWs and Porsches, or the straight six BMWs? How about the era of the ragtop sunroof ?
You see how erroneous this error business really is. Is it an error to even define an era?