On older Cinelli handlebars, the model numbers have 2 numbers, like 66-40, 64-42, etc. The second number is the width c-c, but what does the first number represent. Is it just a model/style or is it also a measurement of some kind. Thanks!

afaik it's just the model number. IE the 64 was straighter than the 65, which was more curly (model 65 was known as the Criterium, so I'm guessing it was a better option for that type of racing?).

All you want to know about Cinelli bars.

https://www.43bikes.com/cinelli-bars-stems.html

Thanks!