The statement that everything was flat earlier in the thread that prior to the late 1800s is not accurate. Antonio Stradivari who lived between 1644 - 1737 so if violins were among the first to have radiused boards from around the late 1800s that can not be correct as the aforementioned gentleman was no longer around but his violins most certainly do have radiused fingerboards.
You better provide some proof. Just because he made violins doesn't mean the fingerboards of his guitars were radiused. With only 5 instruments left, and only one playable, details should be available. Provide them, or I call bullshit.
For example, the only playing Stradivari guitar is The Sabionari made in 1679. Here's a webpage with detailed photos. Note the FLAT fingerboard......
http://www.sinier-de-ridder.com/restaurations/stradivari_eng.html
For your listening pleasure, from YouTube:
https://www.youtube.com/watch?v=cVXD85Lcj1o
Also, Torres is the generally considered the first true modern classical guitar maker, and his most important works date from the 1850's to his death. His guitars had flat fingerboards. Any guitars made before Torres are considered to be baroque instruments, and not true guitars.
Finally - my statement that flat fingerboards is traditional is absolutely accurate.